Change default completion mode to CompletionPopupAuto

Summary:
This makes Dolphin use the "Dropdown List & Automatic" completion mode by default when editing the URL bar.

Given the context of someone typing file paths, I can't see why one wouldn't want to use this feature, unless they don't like how it looks.

Reviewers: VDG, Dolphin, ngraham, elvisangelaccio

Reviewed By: VDG, Dolphin, ngraham, elvisangelaccio

Subscribers: ngraham, elvisangelaccio, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D25391