Update glade files - require gtk3.

Updated the glade files to match the output of recent versions of glade, eg:
decode special characters, resort attributes, etc.

Also, we now require gtk3: we already required gtk3 due to how the code is laid
out, so this should have no real effect on end-users.
Treat time dialog as dialog.

Added the proper hint for the timed-activations window to be treated like a
dialog. This should affect how window managers treat it, especially composing
and tiling window managers, which usually treat the noticeably different.