Enable input context selection

It was not possible to use a different input context than the platform
context. This has been unified across major platforms, depending on the
environment variable "QT_IM_MODULE", the following context is selected:
- null:  default platform context (Wayland in this case)
- empty: no context
- set:   set one, if it exists and is valid (otherwise no context)

Change-Id: I07c6fb339c434f99fc6e092a2e18f00600daa3bc
Reviewed-by: Nedim Hadzic <nedim.hadzic@pelagicore.com>
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>

src/plugins/platforms/wayland/qwaylandintegration.cpp

@@ -128,7 +128,9 @@ QWaylandIntegration::QWaylandIntegration()
     mClipboard = new QWaylandClipboard(mDisplay);
     mDrag = new QWaylandDrag(mDisplay);
 
-    mInputContext.reset(new QWaylandInputContext(mDisplay));
+    QString icStr = QPlatformInputContextFactory::requested();
+    icStr.isNull() ? mInputContext.reset(new QWaylandInputContext(mDisplay))
+                   : mInputContext.reset(QPlatformInputContextFactory::create(icStr));
 }
 
 QWaylandIntegration::~QWaylandIntegration()