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.
It makes more sense to just keep one set of corner textures for server
side view decorations, rather than storing a set for each view. This also
keeps the code simpler when when changing theme parameters.
title_height is a derived variable which needs both config and theme
variables (font height and title padding). The code is tidier calling
post_processing() for this from theme_init()
