Fix bug found by xgettext-gtkbuilder-3.

xgettext-gtkbuilder-3:
	Removed comment about bug.
x_glade.c:
	x_glade_keyword: reverted to the old code.
	init_tags: Use global default_keywords.
	is_tag: Use global default_keywords.

And for homework I have to repeat a hundred times:
I MUST not change code I do not completely understand.
I MUST not change code I do not completely understand.
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