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: In Short View, display icons on top and increase icon size

Therefore, just use Tree View by default, which is much more appropriate anyway. We recently did this for Kate too in D12224: Use Tree View as the default view mode in the Filesystem Browser plugin

Reviewers: RKWard

Subscribers: RKWard-Devel-list

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