Bug 850256 - Add text prediction to Polish keyboard layout. r=rudyl

This is a backport of 9772fddc53e1b7c137058ea247751239ad25803e to the v1.0.1
branch.  The pl.dict filed compiled for the backport doesn't have changes from
bug 838227, bug 851314 and bug 857239 which landed on master nad v1-train only.