Revert "Remove the qmake project files" for snippets

This reverts a limited part of commit ad2da2d27a590333fc89a56fc58700a09c3017b3
that deleted .pro files for snippets compilation.
Some .pro files which contain snippets used in the documentation
itself should be restored.

Task-number: QTBUG-90483
Change-Id: I1b03833c8ff17b5fca43a5b6c5417e8545b1711b
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
(cherry picked from commit 3183673303f6e66a02a7fbd25f4df2da8e979cc5)
Reviewed-by: Nico Vertriest <nico.vertriest@qt.io>

src/concurrent/doc/snippets/snippets.pro

@@ -0,0 +1,3 @@
+#! [qmake_use]
+QT += concurrent
+#! [qmake_use]

src/corelib/doc/snippets/code/doc_src_qplugin.pro

@@ -0,0 +1,4 @@
+#! [3]
+TEMPLATE      = app
+QTPLUGIN     += qjpeg qgif    # image formats
+#! [3]

src/corelib/doc/snippets/code/src_corelib_kernel_qabstractnativeeventfilter.pro

@@ -0,0 +1,5 @@
+#! [0]
+HEADERS += mycocoaeventfilter.h
+OBJECTIVE_SOURCES += mycocoaeventfilter.mm
+LIBS += -framework AppKit
+#! [0]

src/corelib/doc/snippets/resource-system/application.pro

@@ -0,0 +1,3 @@
+#! [0]
+RESOURCES     = application.qrc
+#! [0]

src/gui/doc/snippets/code/doc_src_qtgui.pro

@@ -0,0 +1,3 @@
+#! [1]
+QT -= gui
+#! [1]

src/network/doc/snippets/network/network.pro

@@ -0,0 +1,3 @@
+#! [0]
+QT += network
+#! [0]

src/opengl/doc/snippets/snippets.pro

@@ -0,0 +1,3 @@
+#! [qmake_use]
+QT += opengl
+#! [qmake_use]