#703 Add a way to toggle flags via keybindings (mpv backend)

First of all, thanks for this amazing media player!

I'm using the mpv backend, and it has a feature where one can toggle all kinds of flags via keybindings. For example, I have a keybinding to toggle audio normalization filter. My input.conf looks like this:

F2 af toggle "lavfi=[dynaudnorm=f=75:g=25:n=0:p=0.58]"

This is the syntax: https://github.com/mpv-player/mpv/blob/master/DOCS/man/input.rst

I'd love if one could use these key bindings from within smplayer.