WebView improvements

Actionbar is hidden;
History implemented (by pressing Back button);
Return to PocketCode by pressing PocketCode logo in WebView

Jenkins TestRun:
https://jenkins.catrob.at/view/Catroid/job/Catroid-Multi-Job-Custom-Branch/259/

**Update:** a clean jenkins testrun: https://jenkins.catrob.at/view/Catroid/job/Catroid-Multi-Job-Custom-Branch/260/