Clicking the panel separator now autohides the tree panel. #tsk61

The panel is expanded when hovering over the separator and hidden when
the mouse left the tree.
Clicking in the separator again turn off the autohiding behavior.
Merge branch 'master' into feature/tree-auto-hide
Added hide/tie icon to tree panel. #tsk61
Merge branch 'master' into feature/tree-auto-hide
Merge branch 'feature/tree-auto-hide'