Libretro library frontend core features

[Torinde]

Items from here and here, which are shown as not supported:

• [ ] https://github.com/schellingb/dosbox-pure/issues/42
• [ ] https://github.com/schellingb/dosbox-pure/issues/362
• [ ] Multi-Mouse - Staging uses ManyMouse for that
• [ ] Rumble - useful for games supporting force feedback
• [x] Sensors - not applicable for DOS/Win9x
• [x] Location - not applicable for DOS/Win9x
• [ ] Camera - useful for Win9x
• [x] Subsystem - not applicable for PC emulators, which already emulate various "systems"?
• [ ] Softpatching
• [ ] Username
• [ ] Language
• [ ] Crop Overscan
• [ ] LEDs - guest to show/control on host. The reverse (host status pass to guest) seems supported in https://github.com/schellingb/dosbox-pure/commit/98d103bb6c9df9dd438e310b14aed16f38afea1f.
• [ ] Pointer device - PCjr Light pen, pen/tablet support in PenDOS/Win3x/9x
• [ ] Lightgun device - some games require DOSBox with Gunstick patch

Marked some as not applicable, but maybe somebody else will see a usecase? Some are without a comment - what do those refer to and is it applicable for DOSBox?