Provide global widgets and theming

[Andrea Cimitan]

One of the thing that I dislike using screenlets is the inconsistency/incoherence of many themes and the look of few default ones.
Another thing is slowness on hovering/prelight.

To solve this issue we should follow a global approach, providing *screenlets widgets* to be used in everybody's screenlets.
For example:
----------------------
Screenlet code:
void draw_screenlets_background(ScreenletsColors) {
  cairo...
  cairo...
  cairo...
}

And in the screenlet:
if (theme=systemtheme) {
  screenlet.draw_screenlets_background(this.ScreenletsColors)
}
----------------------

Benefits:
1) Consistency between all screenlets
2) Speed: using cairo (and not svg) reflects in a HUGE speed improvement
3) Writing an own screenlets will be simpler, since you can just do draw_button, draw_background... without wondering about the skin, because the default one will rock :)
4) Port of KDE's Plasma should be easy (even better themes)