fix: use path.get_base_dir() instead of manual split + join

[Svitkona]

This fixes an issue with the profile path resolution in the save function that causes InputPlumber profiles to be saved at the wrong path.

The way the path resolution works right now is as follows: user:// or res:// paths are globalized, the result is split into an array by "/", the last value is popped off, then the remaining parts are joined using "/". The problem is that this strips off the leading /, which turns the result into a relative path. On some setups (especially immutable distros where the path is not writable), this causes errors at runtime when OpenGamepadUI tries to save the profiles:

Jun 15 01:07:07 bazzite gamescope-session-plus[174434]: ERROR: Could not create directory: '/usr/share/opengamepadui/home/kun'.
Jun 15 01:07:07 bazzite gamescope-session-plus[174434]:    at: make_dir_recursive (core/io/dir_access.cpp:178)
Jun 15 01:07:07 bazzite gamescope-session-plus[174434]: Failed to mkdir
Jun 15 01:07:07 bazzite gamescope-session-plus[174434]: ERROR: 1554 [ERROR] [InputPlumber] :0: Failed to save OpenGamepadUI Default (Overlay) to user://data/gamepad/profiles/global_default_overlay_xbox-elite.json
Jun 15 01:07:07 bazzite gamescope-session-plus[174434]:    at: push_error (core/variant/variant_utility.cpp:1024)
Jun 15 01:07:07 bazzite gamescope-session-plus[174434]: 1554 [ERROR] [InputPlumber] :0: Failed to save OpenGamepadUI Default (Overlay) to user://data/gamepad/profiles/global_default_overlay_xbox-elite.json

This PR fixes the issue by using the built-in path.get_base_dir to resolve the base path.

A side effect of this PR is that gamepad profiles may need to be regenerated, since they would be placed at a different path.