process_unicode: Small refactor & linux fix

This moves the unicode input start / end sequences into their own
functions, so keymaps and other functionality can build on it too.

At the same time, it changes how the Linux variant works, to match
reality: CTRL+SHIFT must be unregistered too, and we close the thing
with a Space instead.

Signed-off-by: Gergely Nagy <[email protected]>

quantum/process_keycode/process_unicode.h

@@ -9,6 +9,9 @@
 #define UC_BSD 3
 
 void set_unicode_input_mode(uint8_t os_target);
+void unicode_input_start(void);
+void unicode_input_finish(void);
+void register_hex(uint16_t hex);
 
 bool process_unicode(uint16_t keycode, keyrecord_t *record);