Settings: Mouse sensitivity: added a text input box next to the slider

Closes #2833 (closed)

Added a text box next to the slider for adjusting mouse sensitivity.

There is primitive input validation which disallows TYPING chars which aren't 0.123456789. This doesn't prevent ftos_mindecimals from changing the value to inf or any repeats of . such as 1.2.3.4. IMHO having float string sanitation is above the scope of this MR and should not be implemented here. Perhaps at the engine level instead.