Move drag code from SC.PalettePane to a mixin.

This moves the code that makes SC.PalettePane draggable into a SC.Draggable
mixin. This allows any pane to be made draggable.
Skip handling drag events on draggable panes if the target of the event
has indicated text should be selectable.
Renames draggable pane mixin. Tweaks documentation and comments to make it clear that we're not handling the edge case of panes which appendTo elements other than the body.