Use more modern ImGui input API

[psyGamer]

The main reasoning behind this is that the old API doesn't work well with the mouse position when the window isn't in full screen, causing desyncs between the actual mouse position and the position ImGui thinks where the mouse is.

[swoolcock]

Ok re: our Discord discussion, I didn't realise you had solved the problem by actually rewriting the input system I was originally complaining about. 🤦
I'll take a look at this and maybe update imgui in the process (or in another PR). Note that an imgui update will be a breaking change for the like 2 mods that use it.

[swoolcock]

Code looks fine, will look at doing a release this weekend. I'll probably need to coordinate with Ja so that MappingUtils works immediately.