Restored the "New Layer" button in the NKEY Grease Pencil panel (as was found
pre-2.73) for two cases:
1) When no Grease Pencil datablock is active - This shortcut makes it possible to
add a new layer to start drawing in with a single click again (instead of two
clicks - one to add a datablock, and a second to add the layer)
2) When there are no layers - There is no need to display the UI list in this case,
thus saving a bit more space in the rare cases where this applies.
b9ce21fd0f