Add 'File Properties' to Project plugin's tree context menu

Summary:
This adds a menu item which opens the default File Properties dialog.

It can be used to change access permissions or calculate checksums for
the current file.

Screenshot:

Reviewers: Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, kwrite-devel

Tags: Kate

Differential Revision: https://phabricator.kde.org/D16823