Window offsetting shouldn't assume new windows are automatically focused

[tarek-y-ismail]

Related: #2586, #3693

Our current implementation of window offsetting assumes that the offset is applied relative to the position of the currently focused window. This works when automatically focusing newly opened windows as each window is offset slightly from the one before it. But with focus stealing prevention, that is not the case as multiple windows could be spawned without changing focus.

This might potentially cause clients that don't support xdg-activation to open behind the currently focused window, being completely occluded; or in the case of similarly sized windows, to completely occlude the windows behind them.

A reasonable implementation should take into account all visible windows and make an attempt to position new windows such that they don't occlude existing windows. In case this is not possible (fullscreen application, too many windows open), it would be acceptable to occlude an existing window.