Store Glade files in a GResource instead of installing them.

* ui/glom.gresource.xml: List all the .glade files.
* Makefile_glom.am: Use glib-compile-resources to create a .c file
  with the GResource data, and compile and link it in to the
  Glom executable.
* glom/glade_utils.h: Use Gtk::Builder::create_from_resource() instead
  of create_from_file().

  This simplifies installation.
GimpRuler: Fix a compiler warning