Dragging - Fix dragging breaking via keybind

[johnb432]

When merged this pull request will:

• Previously, if you double tapped the dragging keybind, it would fail to release the claim on the dragged object. This PR fixes that.
• Stopped setting variables unnecessarily. I'm a little afraid of race condition bs that could happen.
• Use local _target variable, instead of getting QGVAR(draggedObject). Imo the updated version is safer, because QGVAR(draggedObject) could be changed for some reason.

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