Doc: Fix QIcon documentation

* Move \externalpage topics to external-resources.qdoc, as that command
  cannot be embedded in other topics.

* Fix references to non-existent function fallbackIconTheme().

Change-Id: I5d08206c53aea9c2d4c6fddf5d04df187b01ef53
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit b5169b5c909d06c61bdc45b09c97c05154043a24)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/doc/src/external-resources.qdoc

@@ -29,14 +29,23 @@
 
 /*!
     \externalpage https://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html
-    \title Icon Theme Specification
+    \title Freedesktop Icon Theme Specification
 */
 
+/*!
+    \externalpage https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
+    \title Freedesktop Icon Naming Specification
+*/
 /*!
     \externalpage https://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#directory_layout
     \title Icon Theme Specification - Directory Layout
 */
 
+/*!
+    \externalpage https://freedesktop.org/
+    \title Freedesktop
+*/
+
 /*!
     \externalpage https://www.khronos.org/vulkan/
     \title Vulkan

src/gui/image/qicon.cpp

@@ -599,9 +599,9 @@ QFactoryLoader *qt_iconEngineFactoryLoader()
   icons: via \l addFile() and \l fromTheme().
 
   \l addFile() is useful if you have your own custom directory structure and do
-  not need to use the \l {Icon Theme Specification}{freedesktop.org Icon Theme
+  not need to use the \l {Freedesktop Icon Theme Specification}. Icons
-  Specification}. Icons created via this approach use Qt's \l {High Resolution
+  created via this approach use Qt's \l {High Resolution Versions of Images}
-  Versions of Images}{"@nx" high DPI syntax}.
+  {"@nx" high DPI syntax}.
 
   Using \l fromTheme() is necessary if you plan on following the Icon Theme
   Specification. To make QIcon use the high DPI version of an image, add an
@@ -1200,10 +1200,8 @@ void QIcon::setFallbackSearchPaths(const QStringList &paths)
     correspond to a directory name in the themeSearchPath()
     containing an \c index.theme file describing its contents.
 
-    \externalpage http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html
+    \sa themeSearchPaths(), themeName(),
-    \title Freedesktop Icon Theme Specification
+    {Freedesktop Icon Theme Specification}
-
-    \sa themeSearchPaths(), themeName()
 */
 void QIcon::setThemeName(const QString &name)
 {
@@ -1222,9 +1220,6 @@ void QIcon::setThemeName(const QString &name)
     \l{Freedesktop} based systems at the moment, and the
     icon theme depends on your desktop settings.
 
-    \externalpage https://www.freedesktop.org/
-    \title Freedesktop
-
     \sa setThemeName(), themeSearchPaths(), fromTheme(),
     hasThemeIcon()
 */
@@ -1282,7 +1277,7 @@ void QIcon::setFallbackThemeName(const QString &name)
     \l{themeName()}{current icon theme}.
 
     If the current theme does not provide an icon for \a name,
-    the \l{fallbackIconTheme()}{fallback icon theme} is consulted,
+    the \l{fallbackThemeName()}{fallback icon theme} is consulted,
     before falling back to looking up standalone icon files in the
     \l{QIcon::fallbackSearchPaths()}{fallback icon search path}.
 
@@ -1301,10 +1296,8 @@ void QIcon::setFallbackThemeName(const QString &name)
     to bundle a \l{setThemeName()}{compliant theme} in one of your
     themeSearchPaths(), and set the appropriate themeName().
 
-    \externalpage https://specifications.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
+    \sa themeName(), fallbackThemeName(), setThemeName(), themeSearchPaths(), fallbackSearchPaths(),
-    \title Freedesktop Icon Naming Specification
+        {Freedesktop Icon Naming Specification}
-
-    \sa themeName(), fallbackIconTheme(), setThemeName(), themeSearchPaths(), fallbackSearchPaths()
 */
 QIcon QIcon::fromTheme(const QString &name)
 {
@@ -1327,7 +1320,7 @@ QIcon QIcon::fromTheme(const QString &name)
     \l{themeName()}{current icon theme}.
 
     If the current theme does not provide an icon for \a name,
-    the \l{fallbackIconTheme()}{fallback icon theme} is consulted,
+    the \l{fallbackThemeName()}{fallback icon theme} is consulted,
     before falling back to looking up standalone icon files in the
     \l{QIcon::fallbackSearchPaths()}{fallback icon search path}.
 
@@ -1341,7 +1334,7 @@ QIcon QIcon::fromTheme(const QString &name)
 
     \snippet code/src_gui_image_qicon.cpp 4
 
-    \sa fallbackIconTheme(), fallbackSearchPaths()
+    \sa fallbackThemeName(), fallbackSearchPaths()
 */
 QIcon QIcon::fromTheme(const QString &name, const QIcon &fallback)
 {