Introduce "Reset Font Size" action, bound to Ctrl+0

Add a title label to the pages. Use QGroupBoxes throughout.
Extend SavedAction to work with checkable QGroupBoxes. Polish UI files,
use common layout for VCS plugins. Performance: Apply only visited
settings pages. Add search keywords.

Task-number: QTCREATOR-26

Based on a window-modal input dialog, so that it appears as a sheet on
the Mac.

Reviewed-by: Norwegian Rock Cat

With the new representation it fits fine, and this is a bit more
convenient for the user.

This is to make some room for embedding the color scheme editing back in
the options dialog.

In preparation of being able to select from multiple color schemes.

Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>
Task-number: 234394

Looks a bit nicer now and the tooltips explain some of the options a bit
better. Header texts have also been simplified.

(cherry picked from commit ad5bdf69)

Looks a bit nicer now and the tooltips explain some of the options a bit
better. Header texts have also been simplified.