Deadkey support

[martonmiklos]

The Hungarian language uses accents "heavily". The SFOS 2.1.4 updated introduced the deadkey base keyboard which features a a separate deadkey button which activates the long accent version of the vovels:

From the presage side it means if the deadkey is activated the predictions continuing with vovels with no accents/short accents should be removed.
In the attached example the "alatt" suggestion should be removed from the list.