…es are opened

When applying changes in a ExplanationPreferencesGeneralPanel dialog,
save the changes to an instance-level ExplanationPreferences object
(that has itself been loaded upon initialisation of the dialog), rather
than to a new object created on the fly. This ensure that any previously
set "last explanation service" setting is preserved.

When initialising the ExplanationPreferencesGeneralPanel dialog, make
sure the "up" and "down" buttons are disabled if no row is selected in
the list of available explanation services (which is normally the case
when the dialog is created).

Add a label next to the "move up" / "move down" buttons in the
ExplanationPreferencesGeneralPanel dialog, to hopefully make it clearer
that the order of the listed explanation services matters.