add the image size (in pixels) to the info module

adds a new variable "show_size", so had to increment the version number
to 3
fix whitespace
fix upgrade function

allow for sequential upgrades
internationalize image size display

as per comment
https://github.com/gallery/gallery3/pull/394/files#r4643737
change "size" to "dimensions" and display unit "px"
Merge pull request #394 from jamaa/module_info_imagesize

Add an option to the info module to display the image size (in pixels) of the current item.

Fixes #2075