o.c.o.converter: use control PV name on converted BoolButton.

When an EDM MessageButton is converted to a CSS BoolButton, use the
control PV of the former as control PV of the latter.

This makes the button disable when the PV is not writeable, which
is consistent with other button behaviour.
Merge pull request #855 from dls-controls/854_boolbutton

o.c.o.converter: use control PV name on converted BoolButton.