WLCS depends on compositor-specific behaviour

[RAOF]

Meta-issue to track all the places in WLCS where we implicitly depend on Mir's behaviour

• position_window_absolute might not work in all window managment schemes #273
• WLCS relies on implicit focus on creation #240
• WLCS assumes that the dimensions of the wl_output are the only popup positioning constraints #279
• WLCS assumes that clicking outside a non-grabbing xdg_popup dismisses the popup tree #280
• WLCS assumes windows are arbitrarily mouse-movable via xdg_toplevel_move #284