Playlist sort widget: reimplement Shuffle "sort" as an action.

Remove now-unnecessary Shuffle handlers in sort-related functions.
Additionally, PlaylistController::moveRows() has been modified to help
with big move actions (validation complexity reduced).

Playlist::BreadcrumbItemMenu introduced to deduplicate logic between
Playlist::BreadcrumbItem and Playlist::BreadcrumbAddMenuButton.

GUI: In the playlist sort widget, the Shuffle menu entry is now
separated from other entries. Activating the entry no longer results in
a "Shuffle" sort level being added.
BUG: 320129
FIXED-IN: 2.8
REVIEW: 110658
CCMAIL: amarok-promo@kde.org