Puts maintab as 'DocumentMode'

This small patch enable maintab to be  in 'Document Mode', this means
basically that it size is smaller and there's not a line separating
the widget anymore, giving the user a more smooth experience.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Add a proof of concept for filtering the Dive List,

Press CTRL+F and a line edit will appear, whenever you write on that will
be used as a filter against all columns. The results are maybe somewhat
surprising in trip mode, but when sorting by another column this shows
some potential.

Hit ESC to remove the filtering.

I need to find a better position to put the Widget, but it's a proof of
concept.

Signed-off-by: Tomaz Canabrava <tcanabrava@kde.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>