place-window-interactively: ignore only ButtonRelease events

emacs for example, maps immediately if we ignore any event...
Mimic CTWM interactive placement features

When placement-mode is set to 'interactively, during the 3x3 grid
move:
- clicking Button2 and dragging the outline will give
  the window its current position but allow the sides to be resized;
- clicking Button3 will give the window its current position but
  attempt to make it long enough to touch the bottom the screen;
- releasing any other button still position the window at the current
  position, as before.

Thanks to CTWM project for its documentation.
Merge pull request #10 from maxatome/master

Interactive placement additions