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
fix error introduced during recent changes to Mackie control support, involving ambiguous enum usage
bump to 3.4
vtl: fix small issue with video source file on export
squash another compiler warning