Add manual setting for external clock frequency

[mrehkopf]

When using the external clock feature of DSLogic, all time and duration calculations were wrong because there was no way to hint the external clock frequency to the application.
This feature adds a manual external clock frequency input to the Device Options dialog. It also shows that the external clock is in use in the Sampling bar (top of window where the duration and sampling rate are set).
The external clock frequency is also shown in the drop down box.

[mrehkopf]

Also, the old range of possible input frequencies is brought back (10kHz upwards) in dsl.h. Not sure if this is a problem, I had no problem using them.