ExtensionManager: Hide ExtensionManagerWidget class in .cpp

And de-pimpl. Smaller overall interface and less indirections.

Change-Id: If6a5c0824581a6478ad6e88410d97e23cc517a53
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/extensionmanager/extensionmanagerplugin.cpp

@@ -50,7 +50,7 @@ public:
         using namespace Layouting;
         auto widget = Column {
             new StyledBar,
-            new ExtensionManagerWidget,
+            createExtensionManagerWidget(),
             noMargin, spacing(0),
         }.emerge();