#98 default time instant timbre setting

[Craig Stuart Sapp]

Time instant layers can be performed with sounds.
These sounds are controlled by a "Plugin Parameters"
window available at the bottom of the layer control
tab for a time instant layer.

By default when a time instant layer is created
or loaded (at least a text layer -- don't know
about svl file loading), the time instant sound
is set to "tap".

It would be nice if there were a way to set what
the default sound will be when a time instant
layer is created. For example, I want to use
"click" as the default (for some reason :-).

It would be convenient to allow a default sound
setting for time instants in the "Sonic Visualiser
Application Preferences" window found in the menu
system under "File->Preferences".