Add more theming / configuration options

[Consolatis]

• [x] (max) menu width possibly combined with calculation of dynamic width based on the widest element within a given menu node
• [x] menu font padding within a single menu item
• [ ] submenu bullet
• [x] button width
• [x] button layout
• [x] button hover background + icon
• other suggestions?

Implementation help: http://openbox.org/wiki/Help:Themes#Theme_file_structure http://openbox.org/wiki/Help:Themes#Button_images

[acoolstraw]

• [ ] menu border
• [ ] menu corner radius
• [x] alt tab border width
• [x] alt tab border color
• [ ] alt tab border radius

[Consolatis]

Related: https://github.com/labwc/labwc-acceptance-criteria/issues/2

[acoolstraw]

• [ ] osd.hilight.* for workspace and alt tab OSD styling