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 :(
fix vari-speed phase (when used for calculation only)

and clarify an old comment.
update external plugin UI handling

amend to 4cdb018 and 1d972d0

override ui_closed() behavior for lv2ui:external
Keep UI around and do not re-instantiate, but simply show it again.
(this is against the original specs but was agreed upon by various
authors and the previous behavior or Ardour.)

kx:external-ui are cleaned up after ui_closed().
protect meters against weird float values
fix typo in IEC-meter 'weird-float' protection.