Output pwm maximum as function of channel frequency

(This builds on https://github.com/TauLabs/TauLabs/pull/979/files. The function of interest is ground/gcs/src/plugins/config/outputchannelform.cpp)

This generates the maximum spinbox values as a function of the channel frequency, saturating at the fixed-point integer upper-bound. Currently, the maximum value supported by GCS is 9999, which can be higher or lower than the true maximum pulse width.