Use TreeView to render the completions (#39).

Use TreeView to render the completions (#39).

This seems to work, but there are some known issues with gtk3 at the moment.
- The tree view widget size could not be set, so that tree items could be
  hidden if there are to many of them in completion
- The active styles are not applied if used with gtk3
- The tree view shrinks to on single item and becomes unusable on some urls of
  the history - seems there are some char in them that break the tree view or
  the column renderer

Not that the configuration for max-completion-items was removed that could not
be applied to show the tree view.