update external plugin UI handling

amend to 4cdb018 and 1d972d0

override ui_closed() behavior for lv2ui:external
Keep UI around and do not re-instantiate, but simply show it again.
(this is against the original specs but was agreed upon by various
authors and the previous behavior or Ardour.)

kx:external-ui are cleaned up after ui_closed().
protect meters against weird float values
fix typo in IEC-meter 'weird-float' protection.
fix keyboard interaction with volume knobs (monitor section)
improve scroll and drag behaviour of volume-controller knob