X11: use sourceid instead of deviceid for input events

[valaphee]

While trying out gamepad device events for X11, I noticed some issues:

• deviceId is useless, as it always is either the value of the virtual keyboard or virtual pointing device, even though the real deviceId is known and supplied via sourceid, which is only used for raw keyboard events.

This PRs goal is to address both issues.

• [ ] Tested on all platforms changed
• [ ] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [ ] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [ ] Created or updated an example program if it would help users understand this functionality
• [ ] Updated feature matrix, if new features were added or implemented