Mesh component highlighting

Functionality to highlight mesh nodes and elements in the GUI (as requested by Agnes Gräbe and various others).

![Highlighting](https://f.cloud.github.com/assets/798445/385995/260c46f4-a699-11e2-9283-bd2284906b2d.png)

Mark element in (1) or (3) and see details in (2)
Mark nodes in (2) and see position in (3)