mode

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

RevBy: Roberto

Instead of directly adding QToolButtons to the tool bar of the
navigation subwidget, use QActions. This allows the tool bar extension
to show them in a menu when necessary.

The icons are inappropriate for in a menu, however.

view has been specified. This allows to use the open editors view
as tabbar replacement also in split mode.

Conflicts:
	src/plugins/coreplugin/editormanager/editormanager.cpp

Details:  - Happened in release builds when opening pro files with
CONFIG scopes and a non-valid Qt version set.

    Task:     180315
    RevBy:
    Details:

    Details:  also accept trailing slashes

Done with mae.

Details:  That hasn't worked in the last few months, since we changed
the QtVersion class from being passed by value to being passed around by
reference.