Add an Openlicht URL preference

mobile/build.gradle

...ain/java/org/openhab/habdroid/ui/PreferencesActivity.java

...in/java/org/openhab/habdroid/ui/RecognitionsActivity.java

.../src/main/java/org/openhab/habdroid/ui/WidgetAdapter.java

...le/src/main/java/org/openhab/habdroid/util/Constants.java

...ava/org/openhab/habdroid/util/RetrofitClientInstance.java

mobile/src/main/res/values/strings.xml

mobile/src/main/res/xml/openlicht_connection_preferences.xml

mobile/src/main/res/xml/preferences.xml