Add support for joypad touchpads

[Nintorch]

• Closes Add support for accessing DualShock/DualSense touchpad finger touch positions godot-proposals#14481

I decided to split my big SDL3 joypad features PR ( #107967 ) into several smaller PRs, this PR is one of them.

This PR adds the ability to use touchpads on supported controllers, such as DualShock 4, DualSense and (most likely) Steam Deck built-in controls (EDIT: apparently not, since "thirdparty/sdl/joystick/hidapi/SDL_hidapi_steamdeck.c" file doesn't have any references to touchpads).

Currently there's no InputEvent that gets fired when a finger appears/moves on a touchpad.

TODO:

• Mark as experimental
• Add a note about Android automatically turning joypad touchpads into a mouse cursor
• Test on iOS later after Add support for SDL3 joystick input driver for iOS #114316 is merged
• Write a proposal
• Make p_touchpad have the default value of 0
• Allow touchpad events to be ignored if joypad input is disabled on unfocused application
• Input events, including a motion event with movement vector (Can be done in a separate PR if required)
• Method for getting the pressure value
• Default value for get_joy_touchpad_fingers method touchpad parameter

TODO for a separate PR:
Input events:

• InputEventJoypadTouchpad (press/release)
• InputEventJoypadTouchpadMotion (finger motion, with initial position and velocity to make things easier)

[Meorge]

Tested with a wired PS4 DualShock and PS5 DualSense controller on macOS, and it works great!

Here's a very minimal Godot project for testing, adapted from the previous PR, to make it easier for others to test as well:
test-joypad-touchpad.zip
(I can put it up on a GitHub repo if necessary, but it's so minimal unlike the adaptive triggers one that it's probably fine this way 😅 )

[Nintorch]

Thank you for your review, AThousandShips! I included your suggested changes into the code!

[Nintorch]

Apparently, SDL only reports pressure values of 0 or 1, so I will probably need to remove the method to get the pressure value, because it's not that useful in this case.

[Nintorch]

Since this PR lacked a proposal I decided to write one: godotengine/godot-proposals#14481

[bruvzg]

Tested on macOS and iOS with DualSense controller, controller touchpad seems to be working as expected.

[Nintorch]

Thank you for your review, bruvzg! :)

[Birdulon]

Apparently, SDL only reports pressure values of 0 or 1, so I will probably need to remove the method to get the pressure value, because it's not that useful in this case.

True for Sony controllers, however Valve controllers do report a range of pressure values. Tested on Steam Deck Desktop Mode (Gaming Mode forces Steam Input to present an emulated xinput controller) on SDL3 main, and the new Steam Controller (codename "triton") with libsdl-org/SDL#15528

[AThousandShips]

Style and docs look good, haven't tested

[Nintorch]

Thank you for your review, AThousandShips! :)