Set visible extent taking rotation in consideration

This fixes the clipping problems but makes all navigation code based
on the concept of "extent" forceably reducing scale (as rotated extent
is now larger than the non-rotated one). The solution would be NOT
to rely on "extent" but rather on scale and center.
Add setCenter method to QgsMapCanvas and use it from the app

This is to reduce the problems with using extent w/out considering
rotation. Fixes setting map center via the Coordinate edit widget
Fix map panning with rotated map
Fix wheel zooming in presence of rotation
Hush oververbose debugging prints