Exorcise beeps from the Colours pane in Gtk.

Exorcise beeps from the Colours pane in Gtk.

The colour list box beeped at the user whenever it found that
something other than exactly one colour was selected. This seems to
happen implicitly in Gtk when the pane is changed. In Gtk1, this gave
you a beep whenever you left the Colours dialog after having selected
a colour from the list; in Gtk2, you additionally got a beep _every_
time you subsequently re-entered the Colours dialog (for reasons I
haven't investigated). Windows was unaffected.

Also, in Gtk (unlike Windows), it's possible for the user to go back
to the state where no items in the list box are selected at all.

For these reasons, stop beeping at the user, and instead blank the RGB
edit boxes as a hint that edits to them would be futile. (Really we
should be disabling them entirely, but the cross-platform edit
controls aren't up to that yet.)

[originally from svn r8074]