Gets emitted when the inline autocompletion is triggered.
The default behaviour is to make the entry display the whole prefix and select the newly inserted part.
Applications may connect to this signal in order to insert only a smaller part of the
prefix into the entry - e.g. the entry used
in the FileChooser inserts only the part of the prefix up to the next '/'.
the common prefix of all possible completions
true if the signal has been handled