ime: render pre-edit text

This is done by allocating cells for the pre-edit text when receiving
the text-input::done() call, and populating them by converting the
utf-8 formatted pre-edit text to wchars.

We also convert the pre-edit cursor position to cell positions (it can
cover multiple cells).

When rendering, we simply render the pre-edit cells on-top off the
regular grid. While doing so, we also mark the underlying, “real”,
cells as dirty, to ensure they are re-rendered when the pre-edit text
is modified or removed.

vt.c

@@ -408,7 +408,7 @@ action_esc_dispatch(struct terminal *term, uint8_t final)
         case '=':
             term->keypad_keys_mode = KEYPAD_APPLICATION;
             break;
-ö
+
         case '>':
             term->keypad_keys_mode = KEYPAD_NUMERICAL;
             break;