Manage the GUI sub-process.
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.
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.