Global Speed Control logic improved
Global Speed Control logic improved

If the globalSpeedControl contains an invalid
value (less than or equal to 0):
1. an alert is displayed explaining the problem
2. its background colour is changed to pink
3. the Start button is hidden.
The speed validity is checked again for the
particular score, when the Start button is clicked
(in beginRuntime()). That code already existed.

notator/assistant-performer,notator/assistant-performer