Support for GtkBuilder UI files.

Fix Savannah bugs #29216 and #29900.

This patch uses switch statements to split support for Glade2 files
and incompatible GtkBuilder files.

The file check is done at the first tag passed by expat (the root tag)
so file ending is not a problem and also xml generic files could
be parsed using this command line:
xgettext -L Glade --keyword --keyword=tag1 ... file.xml

However, no attribute will be stored in the resulting POT file,
only tag content (when there is not any nested tag below it).
Tests for GtkBuilder UI files.
Document GtkBuilder UI files support.