Based on work by Bastien and Brecht in the Node Editor, this adds more
generalized support for selecting items so that click+drag actions on
items (nodes, makers, dopesheet keys, etc.) works as wanted.
Note that this only adds the barebones to support this in other editors,
it's not used yet (will be done in followup commits).
The behavior is supposed to work as follows:
* Clicking an unselected item immediately selects it, and deselects
other items (doesn't wait for release events).
* Click+drag on an unselected item immediately selects it, deselects
others and drags it in one go (don't require selecting it first!).
* Click+drag on a selected item won't change the selection state (and
won't send an undo push) and start dragging all selected items as soon
as the drag event is recognized.
* Clicking on a selected item will still deselect others, but that will
only happen on mouse release, when we know the intention is not to drag
the item.
Included in: https://developer.blender.org/D5979
Reviewed by: Brecht van Lommel, William Reynish
be2cd4bb53