Feature rectangular showcase

Allows changing the showcase circle shape to a rectangle. Does not draw the cling drawable around it, just highlights an element by darkening all elements around it (or any other background that is set in sv_backgroundColor of course).

Rewritten calculateTextPosition in ClingDrawerImpl to find the best placing for the text on all 4 sides.

![example](https://f.cloud.github.com/assets/5778449/1412594/62ee781c-3dd9-11e3-9ac0-27048735785a.png)