GtkBuilder: Make IDs optional

GtkBuilder: Make IDs optional

One requirement of .ui files is that each object must have an ID,
even if it is never referred to or directly loaded from the code.
This makes editing .ui files much more onerous than it has to be,
due to the frequent need to invent new IDs, while avoiding
clashes.

This commit makes IDs optional in the XML. They only need to
be provided for objects which are referred to or explictly loaded
from the code. Since GtkBuilder needs IDs for its own internal
accounting, we create IDs of the form ___object_N___ if not
specified in the XML.

https://bugzilla.gnome.org/show_bug.cgi?id=712553