Enhancement: Enable Load/Store to SD for Signal Generator Configuration

[plfinch]

Add the ability to save and restore Signal Generator Settings to SD card.

Suggested Implementation:

• Use a different filename suffix for saved Signal Generator configuration files versus Spectrum Analyzer configuration files.
• On the Signal Generator Mode display page, split the MODE control at the bottom of the display into 3 controls: MODE, LOAD SD, STORE SD.
• For LOAD SD, present the list of available Signal Generator configuration files for selection similarly to loading of Spectrum Analyzer file dialog and load configuration as requested.
• For safety, Signal Generator configuration loading should always disable output (set LOW OUTPUT to off).
• For STORE SD, prompt for filename entry similarly to Spectrum Analyzer file dialog and save the configuration to SD.
• If no SD card is available, disable or remove the controls.

NOTE: SFor simplicity, the suggested implementation uses two types of files and separate dialogs for loading Spectrum Analyzer and Signal Generator configurations. I would not be opposed to combining the files so when in a LOAD SD user could choose either type of file and effect a MODE change as appropriate when performing the load. I think I actually prefer this.