[udev input] add character and keyboard modifier information to input_keyboard_event()

This issue follows directly from the work the @jdgleaver did on PR https://github.com/libretro/RetroArch/pull/12981. I hope he'll forgive me for copying directly from the notes on that PR in order to make it easier to keep up with the remaining work.

Issue

• Whenever RetroArch is run under X11, keyboard events are handled (i.e. input_keyboard_event() is called) in x11_common:x11_check_window(). When the udev input driver is used, keyboard events are also handled in the driver itself.
• With jdgleaver's commit in today's build, calls to input_keyboard_event() are suppressed whenever an x11-based context driver is active (i.e. whenever x11_common:x11_check_window() is being called)
• The proper solution is to suppress input_keyboard_event() calls in x11_common:x11_check_window() whenever the udev is active - however, this would require the udev driver to be fleshed out such that the missing 'character' and 'modifier' values can be determined. The 'modifier' is easy enough, but I can find no useable reference material for converting a udev keycode into a localised Unicode/UTF-8 representation.

TODO

• Suppress input_keyboard_event() calls in x11_common:x11_check_window() whenever the udev input driver is active. This would require the udev driver to be fleshed out such that the missing 'character' and 'modifier' values can be determined.

Start by tracing keyboard handling through the udev input driver and x11_common:x11_check_window(), focusing on the existing input_keyboard_event() calls. Determine how the udev path can provide character and modifier values, then suppress the X11 call when udev is active and verify that input_keyboard_event() receives the missing information.