Make current icon format setting available via the rest api

[mueller-ma]

Currently it's possible to check the available icon formats and the default setting of basic ui via the rest api, but not the current setting: http://openhab/rest/config-descriptions/ui%3Abasic.
Other clients could fetch the icon format setting of the basic ui and use this as default value. A common source of issues with svg icons in the openHAB Android app is that users change the icon format in the paper ui for the basic ui and think it's a global setting.