Add separator plugin and support for gradient backgrounds.

These commits add support for gradient backgrounds and a separator plugin. Both are supported by tint2conf and work nicely.

Few words about separator plugin: It's there just for eye-candy/clarity purposes. What comes to the eye-candy part; there are few available styles and they are not super pretty, but my work here allows others to provide new patches to add new styles, or improve what I provided.

All code I wrote is available for free :)

Task list:

• Gradients
  • Implementation for 2-color gradients
  • Direction (or should follow panel orientation? not clear)
  • Multiple gradient segments with positions (e.g. at 0% color 1, at 10% color 2 and at 100% color 3)
  • Gradients for mouse over
  • Simplify by allowing only one gradient per background/state
  • tint2conf support
• Separator
  • Implementation in tint2
  • tint2conf support
  • Change encoding from numbers to names ("dots", "lines", "small dots" etc.)
  • tint2conf: change spinner into drop down (combo box)
  • If it cannot be centered precisely, increase size by 1 pixel