Reviewed-by: hjk

And adapt the device selection list.

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

Broke with 61e468a8.

If we are going to be uncertain, then let's try to
be somewhat better stylistically about it.

This happens for example when removing a split editor.

Details:
The fade-in is a lot faster now, to
prevent things from feeling unresponsive.

This commit makes the uglyness of our current default arrow
icons visible. The toolbuttons are also too wide.

This is a fix for the problem, that some users were unable to figure
out how to open projects.

RevBy:    dt

Reviewed-by: Jens Bache-Wiig

Merge-request: 900
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

For this we needed to get rid of using QToolBar.

Makes sure that all files recognized by the CppEditorFactory get the
proper icon.

Also synchronized suffixes to match what is in the xml file.