[Feature Request] Add ability to slow down emulation speed

[Hexadigital]

Is your feature request related to a problem? Please describe.
When developing achievements for games requiring fast series of inputs (such as full combo achievements in rhythm games, or supers / desperation moves in fighting games), it can be extremely helpful to slow the emulation down for testing them.

Describe the solution you'd like
I would love a hotkey for toggling a slowdown speed similar to what RetroArch has - one that can be adjusted depending on the game being devved and the dev's skills (10% slower, half speed, quarter speed, etc)

Describe alternatives you've considered
Currently, the only workarounds I've found are to slow the game down in RetroArch and take states to load into RALibRetro, or to use the Frame Advance hotkey and very painfully input buttons that way.