Fix CAS computation of a selected subformula.

Invoking a computer algebra system program for computing a selected
subformula has never worked (checked with all LyX versions back to 1.3)
and, moreover, in the presence of a selection things go awry.
This commit allows this computation by replacing the selected
subformula with the result of the computation.
Merge remote-tracking branch 'github/build-bot-2.1.x' into bleeding-edge-2.1.x
2014.11.10-00.00