Fix more of bug #5446: Enable to copy the contents of an InsetInfo through the contex...

Fix more of bug #5446: Enable to copy the contents of an InsetInfo through the context menu.

This introduces a new LFUN LFUN_INSET_COPY_AS, which copies a certain Inset to the clipboard. For InsetInfo this is the text that is visible, but this could also replace LFUN_LABEL_COPY_AS_REF, by copying the INSET to the clipboard as a reference, and also a Math inset to copy to the clipboard as latex (including $'s or \[..\]).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@34223 a592a061-630c-0410-9148-cb99ea01b6c8