GUI-wide Settings class

[retiutut]

Is your feature request related to a problem? Please describe.
Not related to a problem per-say.

Describe the solution you'd like
A handful of features should persist once the GUI has been run for the first time and then in each subsequent app instance.

*Example: Expert Mode.

Making this class and doing the two above settings would be a great start.

Data can be stored along with other settings in USER/Documents/OpenBCI_GUI/Settings

Describe alternatives you've considered
Session settings should pertain only to the session.

Additional context
Need to make this class follow the pattern set forth in the the hardware settings class. DO NOT use the pattern set forth in the Session Settings class, since this needs to be refactored.

[retiutut]

Relates to #970 in that there may be some residual settings that need to be in this new GUI-wide class.

[retiutut]

Cyton Data smoothing is a session-specific setting and should remain in sessionSettings.pde class.

[retiutut]

Testing is passing!