input: add new Unicode input mode

This mode is activated through the new key-bindings.unicode-input and
search-bindings.unicode-input key bindings.

When active, the user can “build” a Unicode codepoint by typing its
hexadecimal value.

Note that there’s no visual feedback in this mode. This is
intentional. This mode is intended to be a fallback for users that
don’t use an IME.

Closes #1116

wayland.h

@@ -206,6 +206,12 @@ struct seat {
         uint32_t serial;
     } ime;
 #endif
+
+    struct {
+        bool active;
+        int count;
+        char32_t character;
+    } unicode_mode;
 };
 
 enum csd_surface {