[Feature Request] Leveraging Inputs by Double Tapping

Appears to be a bit more focus on inputs lately, and I've a request that could dramatically expand the capabilities of devices with a limited number of them (like my arcade machine).

Would it be possible to add a double tap option for retropad's digital inputs? I envision that the input mapping would remain as is, but when users go to map the controller binds for a specific game/core, they can bind the double tap for a given input to something different than the single.

Calculated the average time between my button presses to be ~0.15 sec, so if this idea can work, I'd suggest that any time a digital input has a double tap bind, retroarch would wait ~0.2 sec before actually deciding what to do with that input. If a user presses the same button again within that span of time, the double tap bind would be used instead.

Digital inputs with a double tap assigned would obviously have a small amount of additional latency when given a single tap, but many games have buttons that do things that don't require an immediate response ex. weapon change, reload etc... This is especially true for games that would have need of such a feature anyway.

The issue describes adding double-tap bindings for RetroPad digital inputs, with a suggested 0.2-second decision window and separate single- and double-tap actions. No source files, tests, or entry points are named; first locate the digital input mapping and binding-handling code, then define how single taps, double taps, and added latency should be validated.