Put org.jboss.tools.central.themes in category.

Plugin for theming was added in JBIDE-12552, but it isn't shown in any category.

Please, figure out some place (category) to put org.jboss.tools.central.themes.feature.
Now, only way to install org.jboss.tools.central.themes.feature is to install org.jboss.tools.central.themes.feature.source.feature.group and org.jboss.tools.central.themes.feature gets installed as dependency.