input: Hopefully make keyboard/mouse handling more consistent

[xTVaser]

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