Store example files in a GResource.

* ui/glom.gresource.xml: Rename this to ui/glom-ui.gresource.xml
  so we can also have:
* examples/glom-examples.gresource.xml.
* Makefile.am:
* Makefile_glom.am: Generate the gresource source file.
* glom/dialog_existing_or_new.[h|cc]: Get the example files
  from the globally-registered GResource.
This avoid the need to check multiple locations, and avoids the risk
of not checking all possible locations, when distros move the files
around. And this simplifies packaging by not needing the example
files to be installed separately.