When deleting a group, switch to previous group

...insted of the previous configured group.

Closes #304
Fix unit test
patch for iss300

Change the window's layout level behavior
When focusing on a window, bring it to front.
Floating(but not maximized or fullscreened) windows always on top.

Closes #300
Closes #302
Update CHANGELOG
fix AGroupBox.button_press()

Closes #306
cmd_ changes to bring behaviour in line with stack

cmd_up/cmd_down now behaving as cmd_next/cmd_prev and
cmd_shuffle_up/cmd_shuffle_down is introduced.

Closes #307
Add a default timeout for all notifications

Closes #311