Alternative page size + PPI Image export interface for high res graphics

[springmeyer]

Currently the Image export requests pixel dimensions. It would be more useful to provide a drop down of page sizes in units like inches + a resolution (PPI) setting, more like a standard printing interface.

In the case of a PPI higher than 90.1 being requested (the built in pixel size in Mapnik assumes ~90.1 to calculate scale denominators) we should use the paged dimensions + the resolution to determine the correct scale factor.

This would be an improvement upon the current behavior, which after #403, simply passes the project scale factor blindly to the image export after e14ba32.

[springmeyer]

Note: for PDF/SVG mapnik/mapnik#1280 is a blocker for making this live, but we can get this done sooner and just not expose until mapnik is ready.