New look for the dialog headers

[Benedikt Meurer]

I'd like to propose a new look for the headers used throughout the
various dialogs in Xfce, because I think the current way has some drawbacks:

a) it requires a few tricks to change the colors from the hardcoded
styled ones, because the header box colors are not themable separately,
and the default ones (using selected for bg and fg) look kinda weird
with some themes, and

b) there's no visual separation between the header and the content,
which also looks odd with several themes, where the selected bg and the
normal bg colors are very similar (or in general with darker themes).

Therefore I'd like to propose to add a hseparator between the header and
the content and put the stuff into a separate dialog class with style
properties for fg/bg, so the colors (and other visual aspects) can be
changed easily by themes. Also using a separate dialog class allows us
to change the look later easily (currently the create_header function is
kinda limited).

http://foo-projects.org/~benny/tmp/new-header-look.png

The header would simply display the window icon and title (this way all
MCS plugin dialogs would be finally consistent wrt this).

Opinions?

Benedikt