Interrupt based key matrix scanning

[mondalaci]

Currently, KeyMatrix_Scan() is used to scan the keyboard matrices which is suboptimal because it utilizes a much needed busy loop, so we're wasting precious cycles.

As an alternative solution, I implemented KeyMatrix_ScanRow() and called it from a PIT timer interrupt. On Linux Mint for some reason this causes the mouse pointer to go really slow and jump hundreds of pixels from time to time. Weirdly, it works fine on Windows 7 and Android.

By commenting out INTERRUPT_KEY_SCANNER, one can test the my interrupt based implementation.

Ultimately, we should use the interrupt based implementation.

[mondalaci]

Done in ae4d608