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.

meson.build

@@ -90,6 +90,7 @@ executable(
   'main.c',
   'osc.c', 'osc.h',
   'render.c', 'render.h',
+  'search.c', 'search.h',
   'selection.c', 'selection.h',
   'shm.c', 'shm.h',
   'slave.c', 'slave.h',