Manage the GUI sub-process.
The GUI plugin registers itself here.
Should we use the GUI? The input says what the user requested: No -> we never want to use the GUI Yes -> we always want to use the GUI, and throw an exception if it's not available Maybe -> we want to use the GUI iff it's available
Returns a suitable GUI handler if so, or None if we should use a non-GUI handler.
Add a local feed to an interface.
Try to guess whether we have source for this interface. Returns true if we have any source-only feeds, or any source implementations in our regular feeds. However, we don't look inside the source feeds (so a source feed containing no implementations will still count as true). This is used in the GUI to decide whether to shade the Compile button.
List the implementations of this interface in the order they should be shown in the GUI.