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

This means, creating a new project, we immediately have completion for
the ui file. Also adding stuff to the ui file now changes the codemodel
immediately.

Yet todo, are tests under windows and suppressing a warning if the form
contains a toplevel spacer.

Task:     -
RevBy:    Thorbjorn
AutoTest: -

Details:  in all pluginspecs.