Tabs: add property extra-drag-preferred-action
setup_extra_drop_target() allows for multiple actions on the extra drop target. The preferred action can be changed from the result of ::extra-drag-value and using the keyboard modifiers CTRL/SHIFT. We never have a way of seeing what the preferred action is because we don't get the GtkDropTarget in ::extra-drag-drop. Add a property extra-drag-preferred-action for the purpose of getting the preferred action in ::extra-drag-drop.
Fixes: #660 (closed)