The events that the draggable uses to initiate a drag operation
are the same as the ones a click action consumes for clicks.
It is still possible to combine the two, but it's finicky and far
from being straight-forward. To make this easier, add a dedicated
addClickAction() method to draggables that takes care of the
setup before adding the action to the draggable actor.
In the longer term, we'll want to turn DND into an action, and
have something like GTK's gesture group to allow combining actions
that would otherwise step on each other's toes.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2742>
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