Initial commit for Brightness in RGBController API

* Added DeviceHasBrightness() = false; to RGBController as overridable
* Added Brightness to Mode struct
* Added BrightnessSlider to the OpenRGBDevicePage ui and the supporting
code to pass to the RGBController

Commit amended for code style and to split API changes and controller changes out by Adam Honse
<calcprogrammer1@gmail.com>
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