Backport fix for #7593 to branch.

Backport fix for #7593 to branch.

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Original log:

Fix problem first reported by Diego: If you try to view a child document,
the macros declared in the parent do not work. This is because we were
cloning only the child, which then didn't have a parent.

The solution is to clone the whole structure, starting with the master
document. As a side effect, we only clone each child once.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@39599 a592a061-630c-0410-9148-cb99ea01b6c8