Testing: Custom build with input caching, relative input and misc input improvements

[filoppi]

Source, Pull Request and full change list are available at:
https://github.com/dolphin-emu/dolphin/pull/9489/

Stable/finished version of all my input work from the last year before it will be slowly merged.
It should be stable as it's deeply tested so it can be used as any other build, but testing is highly apricated, especially on Linux and MacOS (I don't have a build for these, the things I'm looking for are in the link above).

Main changes:

• Input are now cached once per input frame and then read from cached values. Outputs are now set (cached) once per frame and applied at the end of the frame.
• "Perfect" Relative input support (not hacked in as before) (mapping a mouse to the right analog stick of FPS is greatly improved).
• New input/output focus code.
• Loads of new input/output function expressions (a lot of them for speed run aid and relative input).
• Large polish and improvements to Qt Input Mapping Widgets.
• ControllerInterface devices population safety and robustness.
  

If you are a speed runner, the new input functions might help a lot as you can record and playback input macros, or even hardcoded them inside expressions and trigger them with an input.
This is also great if you want to play GC FPS games with a mouse.

Download Windows binaries/build based on the code above (merged with master on 2nd of May 2021):
https://github.com/Filoppi/dolphin/releases/tag/1