Re-introduce removed GestureAction method

The get_threshold_tigger_egde() method was renamed to fix the typo, but
it obviously broke the ABI. To be fair, nobody in the whole of Debian
was using the symbol, apparently, so it's not like we broke existing
code. Still, it's not nice to break ABI without bumping soname, so let's
put the old symbol back in — obviously, deprecated — as a wrapper to the
newly added one.