Make layout API-interface a bit simpler

Additionally, also fix a bug when creating a window, where as if the
session name was the same as a window that already existed, it would
fail, accidentally using the window rather than the session at a target.

libexec/tmuxifier-new-session

@@ -24,5 +24,6 @@ if [ -f "$layout_file" ]; then
   exit 1
 fi
 
-cp "$template" "$layout_file"
+content="$(cat "$template")"
+echo "${content/\{\{SESSION_NAME\}\}/$layout_name}" > "$layout_file"
 exec "$EDITOR" "$layout_file"