Merge DeviceHandler{,Factory} to SqlCollection (were: plugins)

And move MountPointManager to SqlCollection target, too.
MountPointManager was only used in SqlCollection and DeviceHandlers are
an implementation detail of the "Dynamic Collection" feature - they
should have never been selectable or even visible to users.

Only thing that prevented this change earlier was a nasty bug that
caused statistics data loss when you toggled the Mass Storage Local
Collection Backends plugin. This bug was fixed in 2.6.

All former Local Collection file backends are now enabled by default.

GUI: The "Local Collection Backends" in Config -> Plugins will
     disappear in 2.7, please update Manual/ConfiguringAmarok
CCMAIL: amarok-promo@kde.org