This quiets deprecation warnings on python 3.8. Also throw an exception
when the requested plot does not exist.

When using a lineEdit and hitting Enter, the editionFinished() signal is
sent twice. This is due to a known Qt bug, and so a workaround is to
ignore the second signal.
This bug is fixed in Qt 5.14.
First bug report : https://bugreports.qt.io/browse/QTBUG-40

Ensure the top/bottom and left/right fields of the bounding box widget
in the draw tab of the instrument viewer stay coherent (ie top to the
top etc) even if the values overlap.

Signed-off-by: Jose Borreguero <borreguero@gmail.com>

Using a single squashed commit as the history is hard to follow.

Signed-off-by: Jose Borreguero <borreguero@gmail.com>