JBIDE-18678 Make dependencies to org.jboss.tools.usage optional

@kaloyan-raev What will happen under RHEL7: user tries to start Browsersim - see error message which asks him to install non-existing package. In good variant he will open Browsersim FAQ and see the description of his problem and the way how to fix it.

I like the idea of separating usage to two bundles: API(or util or even better name) and implementation(with dialog). So Browsersim will have strict dependency to util and @kaloyan-raev will have no troubles of reusing Browsersim and CordovaSim.

@kaloyan-raev is it OK for you?