Allow display synth to be used in conjunction with speech

[jteh]

Currently, the display synth is implemented as a synthDriver, so it cannot be used in conjunction with a real speech synthesiser. While the original intent was that this be used for sighted developers who would use it instead of a real synth, it may be useful for sighted teachers, trainers, etc. who want to follow what a screen reader user is hearing on screen. Therefore, it should be made possible for the display synth to be used in conjunction with a real synth.

Implementation Details

• The display synth could no longer be a synthDriver, as there can only be one synthDriver active at a time.
• It would probably have to be incorporated in functions of the speech module where speech is sent to the synth.
  • There are probably several cases where the synth functions are called directly. These calls should be abstracted to avoid code repetition.

[aleksey_s]

please, see my coment on #82.