Prevent automatic opening of child docs because of natbib labels

Prevent automatic opening of child docs because of natbib labels
	* src/insets/insetinclude.h
	(updateBibfilesCache): adjust comment
	(getBibfilesCache): ditto

	* src/insets/insetinclude.C
	(getChildBuffer): new, return the buffer of the child if it exists
	(updateBibfilesCache): update the child buffer only if already loaded
	(getBibfilesCache): scan the child buffer only if already loaded

	* src/buffer.h
	(updateBibfilesCache): adjust comment
	(getBibfilesCache): ditto
	(bibfilesCache_): ditto

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13869 a592a061-630c-0410-9148-cb99ea01b6c8