Fix crash when putting a float around multiple display maths (bug #7974).

Fix crash when putting a float around multiple display maths (bug #7974).

This also fixes other float-related bugs, such as this:

* select two floats in two consecutive paragraphs and insert a new float around them
=> the master float's caption is put into the lower subfloat instead of below the subfloats.

In the long term, doInsertInset needs a thorough cleanup.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@40639 a592a061-630c-0410-9148-cb99ea01b6c8