cb026964b0 changed most instances of
MuteCheckBox as a selector for the button that mutes/unmutes a source in
the mixer to indicator-mute, but left a few instances of the old
selector, especially in variant themes while the base theme got changed.
This lead to competing selections where apparently indicator-mute won
which meant that the variant themes wouldn't override the base.
Changing all instances of MuteCheckBox to to indicator-mute fixes this
and hopefully prevents future uses of MuteCheckBox as a selector
anywhere.
ba6a6bfdcb