[FEATURE] Preview mode for Atlas prints - sponsored by SIGE

This request adds new atlas functionality to the composer, which allows the atlas to be previewed and exported feature-by-feature.

Users can switch on an atlas preview mode and then navigate through the coverage layer using a new atlas toolbar. In preview mode any changes made to the extent or scale of the atlas map are only temporary and do not affect the original composition, thus allowing users to manually tweak each page in an atlas before exporting it.

Printing and exporting of complete atlases has been moved to their own controls, so that individual features from an atlas can be printed/exported one at a time.

This work is kindly sponsored by SIGE (www.sige.ch).

NOTE: The current, temporary toolbar icons will be replaced by new icons which Denis Rouzaud is currently working on.