CppEditor: Use ProjectExplorer's file icon overlays

Replace the own three overlays with those from ProjectExplorer.

Change-Id: I9eceef4b7b321a47d2b7bd3e86bd758395043a37
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cppeditor/cppeditor.qrc

@@ -1,8 +1,5 @@
 <RCC>
     <qresource prefix="/cppeditor">
-        <file>images/qt_cpp.png</file>
-        <file>images/qt_h.png</file>
-        <file>images/qt_c.png</file>
         <file>images/dark_qt_cpp.png</file>
         <file>images/dark_qt_h.png</file>
         <file>images/dark_qt_c.png</file>

src/plugins/cppeditor/cppeditorplugin.cpp

@@ -451,13 +451,16 @@ void CppEditorPlugin::extensionsInitialized()
     d->m_cppEditorFactory.addHoverHandler(new ResourcePreviewHoverHandler);
 
     FileIconProvider::registerIconOverlayForMimeType(
-        creatorTheme()->imageFile(Theme::IconOverlayCppSource, ":/cppeditor/images/qt_cpp.png"),
+        creatorTheme()->imageFile(Theme::IconOverlayCppSource,
+                                  ProjectExplorer::Constants::FILEOVERLAY_CPP),
         Constants::CPP_SOURCE_MIMETYPE);
     FileIconProvider::registerIconOverlayForMimeType(
-        creatorTheme()->imageFile(Theme::IconOverlayCSource, ":/cppeditor/images/qt_c.png"),
+        creatorTheme()->imageFile(Theme::IconOverlayCSource,
+                                  ProjectExplorer::Constants::FILEOVERLAY_C),
         Constants::C_SOURCE_MIMETYPE);
     FileIconProvider::registerIconOverlayForMimeType(
-        creatorTheme()->imageFile(Theme::IconOverlayCppHeader, ":/cppeditor/images/qt_h.png"),
+        creatorTheme()->imageFile(Theme::IconOverlayCppHeader,
+                                  ProjectExplorer::Constants::FILEOVERLAY_H),
         Constants::CPP_HEADER_MIMETYPE);
 }