Grease Pencil (when not in object mode) implements its own selection
opertor. This operator (`gpencil.select`) returns
`OPERATOR_PASS_THROUGH`, then falls though to `view3d.select` which can
toggle object selection (when using shift-click picking).
Removing `OPERATOR_PASS_THROUGH` would fix the object toggling, but this
was added in 62c73db734 with good reason (the tweak tool would not
work then).
Now prevent `view3d.select` from acting on Grease Pencil (when not in
object mode).
NOTE: this could also go into the poll function.
Philipp Oeser