Recents panel: Clear all in navigation bar / pie

Here is a mod that shows clear all button as a part of navigation bar and pie (replaces recents button when recents panel is shown). Both single tap and long press handlers are implemented and follow the existing GB logic. The general idea and drawables used are from PA.

BTW, together with my last pull request, these are my first steps in Android/Java programming so let me know if I should fix something or do anything in a different way or... :)