[UDisks2] Power down drive on remove if supported

Summary:
It is preferable to to do a controlled shutdown prior to disconnecting
and eventually cutting of power to the drive.

BUG: 270808
Closes T9730

Test Plan:
Connect external drive, select "Savely Remove" from context menu
1. if drive has only on partition, fs is unmounted and drive spins down
2. if drive has multiple partitions:
 a) first (not last) partition can be unmounted and remounted
 b) "Remove"ing last partition spins down drive

Reviewers: #frameworks, ngraham, broulik

Reviewed By: ngraham, broulik

Subscribers: broulik, kde-frameworks-devel, ngraham

Tags: #frameworks

Maniphest Tasks: T9730

Differential Revision: https://phabricator.kde.org/D15932