Port GFig plug-in to GAction/GMenuModel

This patch attempts to port the GFig plug-in to use GAction instead of GtkAction. The main files edited are gfig.c and gfig-dialog.c. However, the sensitivity of the "undo" menu option is changed by pretty much every file in the plug-in, so they all had to be edited to be aware of gfig->app.

The code seems to work fine, but as there are a lot of options I want to fully test before removing the Draft tag. I will post an update once I'm confident there are no (new) bugs.

@Jehan Hi! I noticed that the plug-in crashes when I try to do anything to objects (e.g. move them, select them, go to the previous one, etc). This seems to predate my changes though - using the current master code produces the same results. Just to check, do you see the same issue when you run the current master code? Thank you!