Always refer to Button Descriptor for mappings, Trigger Fire/Release methods simplification

[SeongGino]

Removes a lot of redundancies regarding how Trigger Fire/Release methods worked - now it's just the two methods, with an if-check to determine whether to invoke FFB monitoring or not.

Button Descriptors are now always used when referring to which buttons to change function of or press in the Trigger methods; this is essentially prep-work for Custom Mappings.

Offscreen Button mode, instead of being a bespoke thing, now simply modifies the ButtonDesc table so that Trigger's Offscreen Key Code is the same as the A Button's Onscreen Key.

The LightgunButtons library is modified so that the first key (correlating to the index for BtnIdx_Trigger) doesn't process button events, but still handles debouncing logic, since this is all handled by the main method within OpenFIREmain.