Introduces PhoshToplevelManager and PhoshToplevel classes for
managing and representing toplevel surfaces.

Thanks to that:
 - it doesn't match the window to close via its title or app id,
   always trying to close the correct window instead
 - it removes the window from the overview only after it really got
   closed - which is especially noticable in case of windows that
   refuse to be closed
 - it updates the list automatically as the changes happen, without
   having to close and open the overview back again

Closes #37