Fixes: QTCREATORBUG-483

Reviewed-by: Robert Loehning <robert.loehning@nokia.com>

RevBy: Daniel Molkentin <daniel.molkentin@nokia.com>

Offer to copy the project into an alternative root.

Reviewed-By: con

Reviewed-by: kh

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