This commit makes a floating window centered to its leading window if it has
a valid WM_TRANSIENT_FOR property set. Other it's placed at the center of the
screen.
`emacsclient` started with `-c` or `-t` argument create a new frame that shall
not be used to manage X windows.
Also fix some related input focus issues (with some remaining unfixed).
Close#17.
* Make sure `exwm-manage--manage-window-queue` is cleaned
* Improve input focus handling after unmanaging a window
* Remove a redundant call to `exwm-layout--show`
