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/help/help.pro

@@ -4,7 +4,6 @@ include(../../qtcreatorplugin.pri)
 include(../../plugins/coreplugin/coreplugin.pri)
 include(../../plugins/find/find.pri)
 include(../../plugins/quickopen/quickopen.pri)
-include(../../plugins/welcome/welcome.pri)
 QT += network
 CONFIG += help
 DEFINES += QT_CLUCENE_SUPPORT \