[KFilePlacesView] Add drag auto-activation

This way we can perform auto-activation only when dragging ontop of a place, not inbetween. Something Dolphin cannot do as it only gets a QModelIndex and cannot see the place view's internals.

We can also avoid auto-activation when dragging ontop of a section header, again something Dolphin isn't aware of.

Moreover, we can reuse the mount-then-activate logic, shedding further code from Dolphin.

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Obsoletes system/dolphin!341 (closed)

• Dragged a folder ontop of my places, didn't get annoying view changes anymore when actually intending to drag it inbetween two places to add it to the panel
• Dragged a folder ontop of an unmounted device, it would mount and then switch to it after a short delay

With system/dolphin!342 (merged)

@meven @ahmadsamir