theme: support different colored buttons

theme: support different colored buttons

Add the following theme keys:

- window.active.button.iconify.unpressed.image.color
- window.active.button.max.unpressed.image.color
- window.active.button.close.unpressed.image.color
- window.inactive.button.iconify.unpressed.image.color
- window.inactive.button.max.unpressed.image.color
- window.inactive.button.close.unpressed.image.color

As far as I can tell, the openbox documentation does not mention the
entries listed above, but openbox does support them and some themes
do use them.