FullscreenUI: Add analog control to big picture mode

[jasaaved]

Description of Changes

Enables analog stick navigation in Big Picture Mode by mapping left and right stick axis events to ImGui D-pad inputs. While ImGui provides analog navigation, I did not use them due to unintended behavior (e.g., affecting sidebar navigation).

Diagonal navigation has been disabled for smoother, more predictable movement, aligning behavior more closely with Steam’s Big Picture Mode.

Rationale behind Changes

Some users may prefer using an analog stick over a D-pad for navigating menus and game titles.

Disabling diagonal inputs helps prevent accidental movement, especially with analog sticks.

Suggested Testing Steps

1. Navigate Big Picture Mode using the left analog stick and verify it moves the selection up/down/left/right.
2. Hold the analog stick diagonally and verify the selection does not move.
3. Do the same with a D-pad capable of diagonal inputs and verify the same behavior.
4. Verify D-pad navigation still works as before.

Did you use AI to help find, test, or implement this issue or feature?

Yes. I used AI to learn about and implement ImGui navigation events.

[Mrlinkwii]

works as intended ( did not check the code )

[SternXD]

This needs to be added to the other input sources.

[jasaaved]

This needs to be added to the other input sources.

I added analog control to XInput and DInput for Big Picture Mode.

I moved navigation to InputManager's preprocessor so we can use the user's bindings for Big Picture Mode. If the user doesn't have any bindings set up, it will fall back to the default mapping tables in the SDL and XInput source classes.

Since there's no standard layout for DInput, I didn't alter DInput's class. That one needs bindings from the user to work. But if there are bindings, navigation in Big Picture Mode now works where before all inputs were ignored.