Add DBus methods to query what KStars is currently centered on

There seems to be at present no way to determine the focus of KStars via DBus. There is a method called getOption which is curiously not exposed in the DBus interface in org.kde.kstars.xml, and is therefore not visible to clients. Moreover, a generic getOption method has the problem of not knowing which options are available. Therefore, this MR deprecates the getOption method (which does not seem to be used at all and is a candidate for removal) and instead introduces a getFocusInformationXML which returns comprehensive information about the focus as an XML object, consistent with other such DBus methods that return XML.