Make the welcome screen into plugins.

This removes the all hard dependencies to and from welcome screen,
except the one to the core plugin. More in detail:

- Add IWelcomePage to add a tab to the welcome screen
- Move tabs in the modules where they belong
- Enables QHelpManager to open help fullscreen and contextually
- "Getting Started" moves to Qt4ProjectManager
- Projects & Sessions (aka "Develop") moves to ProjectExplorer
- "Community" remains in the welcome plugin for simplicity

src/plugins/qt4projectmanager/qt4projectmanagerplugin.h

@@ -49,6 +49,7 @@ class MakeStepFactory;
 class GccParserFactory;
 class MsvcParserFactory;
 class EmbeddedPropertiesPage;
+class GettingStartedWelcomePage;
 
 class Qt4ProjectManagerPlugin : public ExtensionSystem::IPlugin
 {
@@ -81,6 +82,7 @@ private:
 
     QAction *m_runQMakeAction;
     QAction *m_runQMakeActionContextMenu;
+    GettingStartedWelcomePage *m_welcomePage;
 };
 
 } // namespace Internal