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.

terminal.h

@@ -331,6 +331,13 @@ struct terminal {
         struct primary primary;
     } selection;
 
+    bool is_searching;
+    struct {
+        wchar_t *buf;
+        size_t len;
+        size_t sz;
+    } search;
+
     struct grid normal;
     struct grid alt;
     struct grid *grid;
@@ -368,6 +375,7 @@ struct terminal {
 
         struct buffer *last_buf;     /* Buffer we rendered to last time */
         bool was_flashing;           /* Flash was active last time we rendered */
+        bool was_searching;
     } render;
 };