regression from

    commit 3d652128
    CommitDate: Thu Jul 31 22:14:25 2014 +0200

        fix memory leak around function descriptions

        Found by Lsan.

There are two implementations of getCategory, one (sc) returns a new one each
time (hence the leak fix) and the other (reportdesign) returns a pointer to one
that belongs to the manger (hence the crash).

The code in formula really looks to me to expect that the getCategory return a
pointer that "someone else" needs to look after, i.e. the reportdesign variant
is the more correct so revert 3d652128 and to
fix the leak make the sc own the ScFunctionCategories and just cache them like
the reportdesign one does

Change-Id: Ifd986301a54b4d20449e864697655cd973e0c4df