disconnect AuxSend connection when deleting bus

fixes crash:
 - add bus
 - aux-send a signal to that bus
 - delete bus
 - add another bus
 - aux-send a signal to "another bus"
 - click "Aux Send" button on "another bus"
 - crash
use dedicated buffers for route (and track)

"scratch buffers are by definition scratch and their contents are undefined at all times"
"silent buffers are by definition all-zero and should not be used for real data"

But track & route were using those for actual data; plugins (which may run
in the same thread and may get the same buffers) use them for scratch thereby
overwriting real data.

In particular get_silent_buffers() (used by LadspaPlugin::connect_and_run)
clears the buffer which can holds real data:
e.g. via  Route::passthru_silence() -> plugin1 -> plugin2 (clears output of plugin1)
clean up 44fc92c3 (not a threading issue but sigc++ problem)

RouteUI is not sigc::trackable itself (only derived classes are)
RouteUI::~RouteUI() does not disconnect from signals on destruction.
add name-tooltips to the meterbridge meters
adjust meterbridge label-heigh options
RMS meter: remove unused peak-detection
minimize meter-peak re-draw area
fix stupid copy/paste error in 00f26394a9
another amend to 00f26394a9
LV2 - use scratch buffers to scratch :)
catch assert()
initialize some uninitialized values
fix typo in c212c16eb
initialize two more uninitialized vars..
valgrind ignore glib UTF8 warnings
fix close_allv() sentinel
correct error message
clean up some valgrid "Uninitialised value was created by a heap allocation" wanings
tiny detail.. GUI-callback may try to check vmonitor->is_started() or issue a periodic seek update while it's going away
use zeroed scratch buffers for "silent" plugin runs

Plugins rewrite the buffer data in-place and some plugins
can produce output even when fed with silence.

Hence, during a PluginInsert::silence() run a plugin can
inject data into the "silent" buffers which causes side-effects.

Kudos to Chris 'oofus' Goddard for finding this issue.