Added CycleTransmitMode global shortcut.
Added transmit mode icons to the toolbar, which will show which transmit
mode is currently in use and allow for quickly changing transmit mode.
This addresses feature request #1119.
* update icons on gui init.
* This code was duplicated, fixed.
* Fixed a bug when Cycle Transmit Mode was used, not updating toolbar
* indicator of transmit mode.
Fixed overqualified function declaration.
Changed to a drop-down box.