reduce calls to win-title

rather than calling the script unconditionally per redraw, we now have
a `title_dirty` flag and keep track of when any of the relavent
information changes.

Co-authored-by: Arthur Williams <taaparthur@gmail.com>
Partially fixes: https://github.com/nsxiv/nsxiv/issues/258

examples/win-title

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # Example for $XDG_CONFIG_HOME/nsxiv/exec/win-title
-# Called by nsxiv(1) on each redraw.
+# Called by nsxiv(1) whenever any of the relevant information changes.
 # The output is set as nsxiv's window title.
 #
 # Arguments, "Optional" arguments might be empty: