Separate theme options for osd prompt buttons.

I managed to keep backwards compatibility without really cluttering
the code so here is the patch
http://bugzilla.icculus.org/show_bug.cgi?id=4874 too.

Please keep in mind that this is my first piece of code for openbox
and that I'm not a die hard openbox user (yet), not to tell the patch
was not exhaustively tested. Anyway I think it's pretty much in a good
shape but any criticism will be welcome.

Basically the patch add the following theme options for controlling
buttons in osd prompts:

%%%% colors
%
% for the text inside the button
osd.button.unpressed.text.color
osd.button.pressed.text.color
osd.button.focused.text.color
%
% for the line art around the button
%  (if you don't wan't the box just make box.color = bg.color)
osd.button.pressed.box.color
osd.button.focused.box.color

%%%% textures
%
osd.button.unpressed.bg
osd.button.pressed.bg
osd.button.focused.bg

The buttons can be in three states:

 unpressed: neither clicked nor selected
 focused: selected but not clicked
 pressed: clicked (and of course selected)

I discarded the previous distinction between press and pfocus as in
fact it was only a formal distinction, in that both appearances
mimicked each other in every sense. It think that it was just
inherited from the way titlebar buttons are managed so I decided to
simplify it a bit.

All the options default in a way that preserves backwards compatibility:

osd.button.unpressed.text.color  -> osd.active.label.text.color
osd.button.pressed.text.color -> osd.active.label.text.color
osd.button.focused.text.color -> osd.active.label.text.color

osd.button.pressed.box.color ->  window.active.button.pressed.image.color
osd.button.focused.box.color -> window.active.button.hover.image.color

osd.button.unpressed.bg -> window.active.button.unpressed.bg
osd.button.pressed.bg -> window.active.button.pressed.bg
osd.button.focused.bg -> window.active.button.hover.bg

Notice that a good deal of locs where added to theme.c but in
compensation prompt.c is pretty much simpler now because the
appearances and textures are created while loading the theme.