EogPropertiesDialog: Convert to GtkBuilder template
Initial GResource migration

Embedding UI data files as GResource makes several things easier.
The compiled resources need to be linked directly with the binary
as they won't be available if linked with the embedded libeog.
EogPreferencesDialog: Convert to GtkBuilder template

Also embed the plugin manager into the template directly.
Remove EogDialog helper class

EogDialog has become obsolete with the switch to GtkBuilder templates.