The current event-based approach is very difficult to get right, and it
depends on no events ever being missed. This changes the keyboard/mouse
handling code to a polling-based approach.
Other fixes:
- an issue where modifier keys were not able to be successfully bound
(like Left Shift to `X`)
- improves cursor hiding (except when you use the start menu, this seems
like an SDL issue, see comment)
- Better discarding of kb/mouse inputs when imgui intercepts input
- properly swap bindings when an already set key is assigned, even if it
crosses the distinction of an analog vs normal button
Fixes#2800
Tyler Wilding