Better gyro aiming

Current gyro aiming implementation is very limited. The main feature missing is being able to enable/disable gyro aiming with a button press (needs more options, like toggle or hold). There are many other improvements though, some examples in order of (subjective) importance:

1. Make use of accelerometer values to mitigate gyroscope drifting
2. Flick stick http://gyrowiki.jibbsmart.com/blog:good-gyro-controls-part-2:the-flick-stick
3. Optional acceleration
4. Optional tightening threshold (to replace current flat threshold)
5. Aim assist? heavily dependent on the game content, should probably come as a separate script, if it is at all possible to implement well for Morrowind

Good guide on how to best implement gyro aiming: http://gyrowiki.jibbsmart.com/blog:good-gyro-controls-part-1:the-gyro-is-a-mouse

Prerequisite issue: #6570