Add Russian-Tatar United keyboard layout

This layout is the second (last) step to Russian-Tatar united keyboard layout described here: https://800147.github.io/rtu/en

This layout adds Tatar letters to Russian layout above their paired letters making it simple and familiar to write Russian text and possible and easy (though not so comfortable) to write Tatar text.

This layout is best solution for those who mostly use Russian language but sometimes wants to use Tatar without need of having two separate layouts. While best solution for those who mostly use Tatar language is to use standard Tatar layout.