Refine fix for bug #7183.

Refine fix for bug #7183.
Using updateMacros() entails a performance hit when loading a document
with really a lot of macros. So, revert to the original strategy of
tracking macros at creation time. In order to also account for macros
defined in a child document, the child is now loaded by the include inset
at construction time instead of after the master has finished loading.
This strategy mimics what updateMacros() was accomplishing without
incurring in its drawbacks, such that loading time is practically
unchanged even with a thousand macros.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@36968 a592a061-630c-0410-9148-cb99ea01b6c8