Centralizes code to return a possible rotated map area boundaries.
Makes use of the new method from the grid decoration.
Adds new testcase for QgsMapSettings, including test for visibleExtent,
mapUnitsPerPixel and the new visiblePolygon method.
When layer set is locked in composer map, also the styles associated with layers are kept.
As a result, a composition can contain maps of the same layer just with different styles.
Map settings and map renderer jobs have been improved to support temporary override of layer's style.
It is possible to pass to map settings which layers should have overridden style and map jobs
will temporarily replace the style (just for the rendering).
