Fix oddities when replacing VST-presets.

Fix oddities when replacing VST-presets.

VST used the count of available of presets as URI:
 - add 2 presets (1,2)
 - remove first, add another one -> two presets with same URI (2,2)

PluginInfo::get_presets() simply lists all (name only) in a vector.
Plugin::find_presets() uses the URI in a map (unique by URI).

..various ensuing bugs: eg. Plugin::remove_preset() looked up by name,
but didn't check for NULL.