Use dispatch_properties_changed() instead of GtkEditable to watch for
changes to the to-be-completed text. This is necessary because various
functions in GtkEntry don't use the interface vfuncs and one only
becomes aware of them via notifications. I'm not sure this is intended
behavior, but it's how it works today.
Also, use the same code for triggering in all situations.

What the code does is this: It looks at the part of the text in front of
the cursor (or selection) and completes for it. Once the directory has
been enumerated, inline completion is activated. Note that popping up of
the completion popup is completely handled by GtkEntry.