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

@kaloyan-raev this code was added to set gtk3 as a default value for RHEL7 users. It was done because RHEL7 has no webkitgkt libs(which is needed to run browsersim). So, if your changes will be applied, ZendStudio rhel7 users will not be able to run browsersim by default.

How to avoid this problem:
1. Use browsersim's own implementation of OS detection. This will be a code duplication:), but everyhing will work.
2. Move OS detection from usage to separate plugin(or even to jar library). It will help you, but @alexeykazakov don't think it's a good idea.