Treeportlet: define context-url data object instead of context-path.

This is a follow up of #596: During the changes to support virtualhostmonster configurations, the tree data has changed from path to url. But to adjust the data attribute, was forgotten.

Additional i've added some small tests for the treeportlet itself.
Merge pull request #605 from 4teamwork/pg_treeportlet_current_selector_fix

Treeportlet: define context-url data object instead of context-path.