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.
fix keyboard interaction with volume knobs (monitor section)
improve scroll and drag behaviour of volume-controller knob
LV2 external UI handling - revert to <= 3.3. behaviour

Don't ever clean up external UI plugins that use the LV2plug.in URI.
only free the memory of the last instance when the plugin is removed
or ardour closes.
LV2 extUI handling - amend to previous commit.
add std:: to clarify use of llabs on OS X
bump to 3.4
revert to --backtrace enabled by default, since the original goal of this change was addressed in other ways
Revert "bump to 3.4"

This reverts commit ad7ab8f09afe1eac025a6da87a45c83ada15544d.
updated .po files from waf i18n, plus an updated gtk2_ardour/de.po from Edgar Aichinger
Merge branch 'master' into windows