Use Tree view for the file browser instead of whatever the default view happens to be set to

Summary:
RKWard's file browser currently relies on the default view provided by `KDirOperator` for its file browser panel rather than defining its own preferred view. This is discouraged because:
- The current default view ("Short view") is probably not ideal anyway, as it will result in a horizontal scrollbar
- We are preparing to change the default to something much less appropriate for your use case; see {D12326}

Therefore, just use Tree View by default, which is much more appropriate anyway. We recently did this for Kate too in {D12224}

Reviewers: #rkward

Subscribers: RKWard-Devel-list

Differential Revision: https://phabricator.kde.org/D12358