free memory of external plugins on GUI close (major mem leak).

There is at least one known plugin that has a problem with this
(custom thread race condition) but it also crashes in other hosts:
http://www.drumgizmo.org/wiki/doku.php?id=bugs&do=showcaselink&showid=8&project=drumgizmo

tested to be working with various nedko, falktx and x42 plugins
add LV2 support for kx:external-ui#

the ABI of http://kxstudio.sf.net/ns/lv2ext/external-ui# is identical to
http://lv2plug.in/ns/extensions/ui#external
It just adds a distinction between #Host and #Widget for cosmetic reasons.
Sadly some newer plugins are released with only support for kxstudio URI :(