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

search.c

@@ -18,6 +18,7 @@
 #include "render.h"
 #include "selection.h"
 #include "shm.h"
+#include "unicode-mode.h"
 #include "util.h"
 #include "xmalloc.h"
 
@@ -993,6 +994,10 @@ execute_binding(struct seat *seat, struct terminal *term,
         *update_search_result = *redraw = true;
         return true;
 
+    case BIND_ACTION_SEARCH_UNICODE_INPUT:
+        unicode_mode_activate(seat);
+        return true;
+
     case BIND_ACTION_SEARCH_COUNT:
         BUG("Invalid action type");
         return true;