Also move Designer::Internal::FormWindowEditor ->
Designer::FormWindowEditor.

- Saving to file only takes modified data into account.
- Search functionality does not exhaust memory anymore.
- Searches can be canceled. This required updating the IFindSupport
  interface and all classes implementing it. No functional changes
  were done in those.

Reviewed-by: mae

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

The margin on the other panels is being set to 4, while the search panel
used the default margin.

Reviewed-by: Jens Bache-Wiig

 - show side-by-side if possible
 - always show side-by-side
 - always show full help

The default is "show side-by-side if possible". It's currently considered
possible if the editor is at least 800 pixels wide.

There were both variants, with and without extra semicolon.

Task-number: 254695

Task-number: 253365

Task-number: 253365
Reviewed-by: kh