SetInputFocus to the i3lock window to force-close context menus (#155)

SetInputFocus to the i3lock window to force-close context menus (#155)

When grabbing the pointer/keyboard fails, a new code path is activated, which:

1. Uses the standards-compliant _NET_ACTIVE_WINDOW root window property to
   determine the window to restore focus to.

2. Sets the input focus to the i3lock window, thereby possibly force-closing
   open context menus (works with e.g. Google Chrome, does not work with
   e.g. thunar, gedit).

3. Upon exiting, restores focus to the window from step ① by sending a
   _NET_ACTIVE_WINDOW ClientMessage to the root window. Note that this step
   requires https://github.com/i3/i3/pull/3027 in i3 to not mess up focus.

fixes https://github.com/i3/i3lock/issues/35