Use KMessageWidget for error- and information-messages

See http://agateau.com/2011/04/21/kde-ux-2011/ for more details.
This simplifies the statusbar in Dolphin a lot and allows to
do a proper eliding in case if status-messages are too long: In
this case a tooltip will show the whole status-message (thanks
to Wolfgang Mader for the initial patch!).

There is still missing some finetuning but the general approach
seems to work quite nice.

BUG: 249638
BUG: 245618
BUG: 146533
FIXED-IN: 4.9.0