Makes the feature tree work again on Macs

Vector-api branch killed the feature tree feature because ctlr-click is not usable on Macs (see snipped at [1])

This PR adresses the issue. We are now using the mac Key instead of the ctlr Key on Max (as in RE2).

[1] http://docs.closure-library.googlecode.com/git/closure_goog_events_browserevent.js.source.html#line340