Using more *::Constants::ICON_* where it makes sense and
wrapping the file names into QLatin1String where they were
missing.

The increased usage of the ICON constants needed a few more
cross plugin includes of *constants.h, here and there.
I think that it is OK, since the dependencies were alredy there
icon resource wise.

Uis in the core plugin should not use icons from the help plugin.
Also, let's use the Core::Constants::ICON_* instead of duplicated
file name strings.

Reviewed-by: con

Now, ctrl+w closes all editors, as does "X" button in the toolbar.

Reviewed-by: mae

Also fixes a problem on Mac Cocoa with not being able to use the close
editor button.

Reviewed-by: Lasse Holmstedt

Reviewed-by: kkoehne

- Remove shared subwindow reparenting from EditorWidget, make EditorWidget
  inherit FancyMainWindow. and use just once instance of it instead of
  per-editor ones.
- Embedded FormEditorStack into EditorWidget as a centralwidget.
- Changed FormWindowEditor's base class from IEditor to SharedTools::FormWindowHost
  (Widget) to be embedded into FormEditorStack (no need to be an IEditor),
  Remove Designer::Internal::FormWindowHost which had little functionality.
- Add Design Mode widget to FormEditorW which has FakeToolBar and EditorWidget
  (single instance) in a vertical layout.
- Removed ProxyAction class handling dock view toggle actions of several EditorWidgets
  (no longer necessary since there is just once instance). Moved "View menu" to bottom.
- Started to make FakeToolBar work as a single instance listening on changing
  xml editors
- Include-file/slot connection clean-up.

Moved Qt Designer to open from Design Mode. Also, Design mode is now global,
and created in coreplugin. Other plugins can register themselves to it.