Move button construction out of pluginInit
Move plugin buttons within instance context

This makes it possible to call instance methods on the current plugin
from event handlers, without the additional indirection through
window.annotator.
Simplify setVisibleHighlights function

setVisibleHighlights was previously two methods disguised as one, behind
the "notify" parameter. This commit moves all the code related to
actually changing local state into the onSetVisibleHighlights handler,
making the setVisibleHighlights function responsible solely for firing
the event.
Guest#setVisibleHighlights will always toggle highlights
Merge pull request #1622 from hypothesis/simplify-highlights

Simplify highlight visibility toggling.