[FIX] web: boolean button terminology is optional

Make sure the widget won't crash when no `terminology` option
is passed.

Also for the translation extraction system to work, each literal
term needs to be enclosed in its own _t() call:

  // No
  var trans = _t(term || "Default term");

  // Yes
  var trans = _t(term) || _t("Default term");

The _t() function supports `undefined` arguments so the second
form is equivalent to the first form at runtime, but supports
proper term extraction.

Finally, simplified the implementation a little bit,
requiring `hover_true` and `hover_false` "action descriptions"
to be provided explicity in custom terminologies.
Previously we tried to reuse the values from `string_true` and
`string_false`, but they describe a state rather than an action,
so they are hardly ideal.

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