[Mousepad 0.5] Restore the use of the file accels.scm

Fixes #73 (closed).

There is a little difference in action naming, but it should not disturb the user, and the feature should behaves the same, although there is some more work under the hood.

The commented entries of the old file will be replaced without warning, but the uncommented entries will be kept, as was the case before: this is gtk_accel_map_save() behavior. This should allow the user to adapt its custom accels to the new syntax quite easily.

NB: I also added a TRACE macro to mousepad/mousepad-private.h for personal testing (in addition to PRINT_LOCATION).