Introduce last_strip_mode option

Like the other similar options, this will be used to restore the state
of the radio button in this future dialog.
Introduce BrowserStripper dialog

This is a shell right now, but will be invoked when a user requested to
strip audio off of cards that they have selected in the browser.
Allow menu actions to suppress shortcut key

This will be used for the new menu item for removing audio, which will
not have a shortcut key.
Wire new remove audio menu into card browser

This is done by extending the already-existing hook and updating the
triggers to disable/enable it.