Accept closures in highgui_set_mouse_callback

[vadixidav]

In highgui_set_mouse_callback currently it is not possible to pass it a closure. By storing an arbitrary dyn Fn type in the Box inside its definition, it should be possible to accept an arbitrary closure to ease callback handling.

[Pzixel]

Making <F: FnMut> would be enough.

[vadixidav]

@Pzixel That is true, but we then have to turn it into a dyn FnMut so that we can pass it to C as a single pointer type since the type is erased. We will find out if we can use FnMut based on if any data races can occur in the code. I think there is a good chance we can accept FnMut. We can only do that if the C code never creates a data race. If it does then we will need to use Fn. If C makes a cross thread data race then we will also need to force it to be Send and Sync.

[vadixidav]

I actually already fixed this in #117. Once that is merged we can close this.