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

It was trying to build the ProjectExplorer before the Welcome mode,
and failing.

Reviewed-by: Thorbjørn Lindeijer

Missing dependency in plugins.pro, sometimes breaks multi-process build.

This check is no longer needed since the necessary private headers are
now part of Qt Creator.

Reviewed-by: Friedemann

RevBy:    - hjk

Note that <Esc> is called <Shift-Esc>.

Details:  Be warned, this is not usable at the moment.