Remove remaining stock items

This removes the arrow on the 'Continue' button in the multiple album
dialog partly to conform with the recommendation that buttons just
have a label describing their action and partly to avoid having to
deal with the RTL variant of the icon. The Alt+N accelerator on
'Cancel' in the infobar is non standard but avoids a clash with the
album composer entry. Alt+P for Pause is non standard but avoids a
clash with the album artist entry and gives a consistent accelerator
for Play/Pause. The disc number entry was missing an accelerator so it
is assigned Alt+D.

https://bugzilla.gnome.org/show_bug.cgi?id=705131
Ensure Extract button is a constant size

If the translation of 'Stop' was longer than that of 'Extract',
'Pause' and 'Play' then the extract button would change size when it
was clicked. Fix by adding fake buttons with both labels to a size
group as for the play and select all buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=706188
Disable search in track listview

The search doesn't work as it's not set up and doesn't serve any
useful purpose so disable it.

https://bugzilla.gnome.org/show_bug.cgi?id=706394