Add angle indication

[VassilyDev]

Hi. I think that a great improvement would be the addition of an angle indication.
For example, I need to fine tune the angle of the servo before flashing my arduino, and doing that with this app would be very convenient.

[kaklik]

Hi, there exists another variant of servo tester application which has angle indication implemented. However, the conversion of an angle to a PWM pulse width is not straightforward, as different servos have different angular ranges for the same pulse width range.