Port ImageMap Plug-in to GAction/GMenuModel

This attempts to port the last remaining plugin, ImageMap, from GtkAction to GAction/GMenuModel.

While working on this, I noticed that many of the icons did not show up in 2.99.14 (Windows). When I checked, I saw that the icon strings abbreviated the name to "imap", while the files themselves had the full "imagemap" prefix. When I renamed the strings to the full title, the icons reappeared. If anyone on other platforms notice the icons disappear after the renaming, please let me know!

NOTE: The entire plug-in's code is a mess with regards to coding style. I fixed what I could in the relevant areas for the GAction port, but a lot more work is needed.