Make sure that we remove the exit button from the log output tab,
regardless of its position in the tab bar.

These previously changed in the OptionsWindow.ui design file.

Simplify the code by removing the corresponding hard-coded definitions.

Add a caption to the dialogue widget and the filter selector.
Default to the location of any previously selected instrument model
file.
Don't unset the reference instrument model file selection if the
dialogue widget is cancelled.
Remove redundant setScaledContents calls.

Remove a setText from UIOptionsWindow.browse_for_reference_model because
it is now redundant.
Also tidy up the wording of some widget labels.

Thanks LGTM!

Also correct some usages of a renamed attribute.

Serialise saved options to a JSON file.  Options to be saved are
determined by searching all the attributes of a UIOptionsWindow
instance.  If the type of an attribute matches a list of recognised
user-input widget types, then we record the value of a specific
pre-determined property of each widget.  For now, the recognised
user-input widget types are QCheckBox, QComboBox and QLineEdit, with the
corresponding properties 'checked', 'currentIndex' and 'text' recorded.

Use QtCore.QObject.tr instead of QtCore.QCoreApplication.translate.
Translate all user-facing strings.  Rationalise some capitalisation.

Also make it so that a UIOptionsWindow without a parent falls back to
logging with a simple print call.

Also add some handy placeholder text in some of the options widgets.