The current widget has a very generic name "EditDialog" - rename it to something
that reflects better what it is for.
Implement it is as a full GtkWidget using GtkTemplate.
Move some of the logic from inside the dialog to the code that creates the dialog
so less data is passed around.
Store properties using functions instead of g_object_set_data - that passes
everything as a gpointer which is easy to make a mistake with.
