Support for non-SDL mouse injection

[extherian]

Hello, I opened this issue already on mupen64plus/core but I believe this is a better place for it.

According to this post on the M64P github, Mupen64Plus has no API for injecting mouse input in the same way as it does for keyboard input.

This means that none of the current Mupen64Plus frontends (M64P and M64Py) can capture mouse input because the frontends use Qt, not SDL.

Please consider adding an API to support this feature. At the moment I have to use PJ64 to get mouse input for shooting games, and I'd rather use a Mupen64Plus frontend if possible.

[StenApp]

I like to have mouse support too.
I use OpenEmu which uses Mupen64plus for N64 emulation.
What is needed that we can use the mouse?