GPencil UI: Streamline the data panel workflow

GPencil UI: Streamline the data panel workflow

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.