Rename THEME_ICONS to THEME_IMAGES

Reason:
THEME_IMAGES is better name because it also contains config line
for wallpaper
Add default font specification
font and font-size can be configured now
rename font[-size] to THEME-FONT[-SIZE]
Add zenburn (high contrast) theme
Rename font variables (cmake)
better readme
Merge dev/zenburn-high_contrast -> devel
Merge dev/font -> devel
Merge dev/readme -> devel