Remove the Glade UI files

We should move all of the UI building stuff from the Glade XML file into code at some point to be more consistent with other native apps.

It would also help with issues like the one where the preferences window doesn't load when running from the build directory.

Start by locating the Glade XML file and the code that builds the preferences window. Determine how all UI construction is currently divided between the file and code, then verify that the preferences window loads when running from the build directory and that the native app UI remains consistent.

Written by the indexing model from the issue text.