Revised thumbnail loading...

- Only load the thumbnails that are currently visible in the window
- Unload thumbnails that are leaving the visible area
- Much less memory needed, but scrolling is now slower
- This also unintentionally fixes issue #86

commands.c

@@ -470,7 +470,8 @@ bool ct_reload_all(arg_t a)
 {
 	tns_free(&tns);
 	tns_init(&tns, files, filecnt, &fileidx, &win);
-	return false;
+	tns.dirty = true;
+	return true;
 }