search: wip: re-direct input while searching, and build a search buffer

This adds a new state, 'is_searching'. While active, input is
re-directed, and stored in a search buffer. In the future, we'll use
this buffer and search for its content in the scrollback buffer, and
move the view and create a selection on matches.

When rendering in 'is_searching', everything is dimmed. In the future,
we'll render the current search buffer on-top of the dimmed "regular"
terminal output.

search.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <xkbcommon/xkbcommon.h>
+#include "terminal.h"
+
+void search_begin(struct terminal *term);
+void search_cancel(struct terminal *term);
+void search_input(struct terminal *term, uint32_t key, xkb_keysym_t sym, xkb_mod_mask_t mods);