Add user visible info to PluginDescriptor

[alanz]

At the moment a PluginDescriptor is defined as

data PluginDescriptor = PluginDescriptor
  { pdCommands        :: ![Command]
  , pdExposedServices :: ![Service]
  , pdUsedServices    :: ![Service]
  }

Add some fields to be used in the IDE UI as follows

data PluginDescriptor = PluginDescriptor
  { pdUIShortName     :: !T.Text
  , pdUIOverview      :: !T.Text
  , pdCommands        :: ![Command]
  , pdExposedServices :: ![Service]
  , pdUsedServices    :: ![Service]
  }

The pdUIShortName should be displayed in menus and the like, rather than the PluginId.
The pdUIOverview can be displayed in any kind of "more information" scenario about the plugin.