Based on user feedback, I've made some changes to the Grease Pencil UI (most notably in 'Time Editing' facilities).
* 'Edit Timing' button gone
* Pin button and '<Grease Pencil Data' string gone from Action Editor
* Action Editor in 'Grease Pencil' mode now displays all grease-pencil datablocks for current screen.
* AE: GP-Datablocks are drawn like 'groups', with an expand/collapse button to show/hide layers. Its name shows the type of space it comes from, and shows indicative status info (i.e. for 3d-view, it shows view-angle)
* Added refresh calls for action editor after editing relevant data.
I haven't tested all tools yet, but most should be stable.
Also, I've removed some unnecessary buttons, and added a few tooltips. There's also some experimental code to try to get clearer indication of 'active' layer.
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