* Move m_key member of Effect into Plugin
* Pass key to Instrument ctors and instantiaters
* Add pluginKeys to all plugin selector widgets, and let them pass the keys
when instantiating the instruments; or, if the keys must be passed over
threads, pass the keys to the Engine using `Engine::setDndPluginKey()`
* As instrument plugin libraries now also need to get their key passed, their
second argument, which was always the same as the first, is now used to pass
the sub plugin keys. This affects *all* instrument plugins.
* Plugin.h: Add more virtuals to `SubPluginFeatures` in order to draw logos
and images into instrument selector widgets
* LadspaSubPluginFeatures: Implement the `displayName` virtual because the
new behaviour to resolve displayNames is to first look at the
SubPluginFeatures, which, without override, returns the superior plugin's
name (Plugin.cpp)
Additional:
* PluginFactory.h: Allow setting up search paths without discovering plugins
yet
* Plugin.h: Add full documentation (should be checked)
