[Feature Request]: Ability to set custom display refresh rate depending of Region

Mesen have a nice option to force user-specified refresh rate in fullscreen mode.
User can select it from drop-down list of refresh rates that your GPU/Dispaly support:

Nestopia had "auto" refresh rate function in fullscreen depending of region.
For NTSC emulator try to set 60 or 120 Hz, prefer higher value, if hardware supported it.
For PAL/Dendy emulator try to set 50 or 100 Hz, prefer higher value, if hardware supported it.
It was really nice when i had CRT, which supported 60-75-85-100-120Hz

I guess nice idea is combine this methods.
To make an separate "fullscreen preferred refresh rate" option.
Two drop-down menus where user can set prefferred hardware refresh rate:
one for NTSC, second for PAL (for PAL/Dendy if system=NES)

Fortunately, lastest modern LCD displays have wide range of refresh rates native support, usually 50-144Hz
as well, as AMD FreeSync or Nvidia G-Sync technologies for automatic refresh adjustment.

No file, test, or entry point is named. Start by locating BizHawk's fullscreen display settings and the code that detects the active system region and available display refresh rates. Done means separate NTSC and PAL/Dendy preferences can select a supported rate, prefer the requested higher rate when available, and apply it in fullscreen mode.