Bring back toolbar auto-stacking (toggleable)

[Nazalassa]

A long time ago, toolbars automatically stacked below each other when opened. Then came r9796 and the movable menubar. Now toolbars don't automatically stack aymore (which, for some people, was a good thing).

This patch adds this behaviour back. It takes menubar position into account and can be toggled off in the display settings.

[prissi]

That was stacking any window which had not remembered coordinates. I changed it a little.

Also, the stacking works only for the first time. I changed that as well in r11765.

It would be nice, if they would search for a matching hole too, once some hole is above a toolbar ...