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

Usage uses the LinuxSystem to track OS names.
Usage is the top bundle in JBT right now. So we have to create a new bundle (or even component) if we want the same LinuxSystem to be available for Usage and browsersim.
As an option, we could separate Usage API and Usage implementation. So the Usage API bundle will be a hard dependency and Usage implementation (including the dialog) will be optional.