OverviewMap control

This replaces both the old #822 and #2444.

The following fixes were made so far:

 - use map `postrender` instead of view `resolution`, `center` and `rotation` property changes, as suggested by @tonio.  The small box doesn't follow the animation of the map (i.e. while doing a pan/zoom), because the extent of the main map is always the same while animating, i.e. is equal to the end result.  More about this.
 - listen to the map view change and act accordingly, as suggested by @elemoine and @tschaub .

What's remaining to do, which includes all comments from #822 and #2444, in my personal order of priority :

 - (bug) defining the control with `maximized: false` no longer works
 - (task) follow the new button structure
 - (enhancement) use map corners instead of the extent as a way to get draw the box to allow it to animate smoothly while the map is animating.
 - (task) add tests
 - (task) support dragging the box to change the main map view (it would be really nice to have this)
 - (enhancement) support rotating the box instead of overview map (this could be left undone for now)