transmission: bump from 2.52 to 2.60

2.60 is the newest of the gtk2-compatible versions. (Transmission >=
2.61 requires gtk3.)

* Add inotify as input to not have transmission-daemon poll the torrent
  auto-load directory.
* Add file as input to fix a small build time issue (non-fatal).
* Remove unneeded gtk configure flags (support is auto-detected).
* icon-theme.cache is only generated when building with gtk, so use
  rm -f to not error out when building without gtk.
* Use gtk ? null instead of gtkClient flag because it is simpler.

Build and runtime tested with and without gtk support.