Create the UIObject interface

It is implemented by every visible object and serves as an abstraction, 
simplification.

Also: there only is one central sharedPreferences object (in 
`Functions.kt`). That makes the code way more readable.

app/src/main/res/layout/list.xml

@@ -45,7 +45,6 @@
                 android:layout_marginRight="8dp"
                 android:gravity="center"
                 android:includeFontPadding="true"
-                android:onClick="backHome"
                 android:paddingLeft="16sp"
                 android:paddingRight="16sp"
                 android:text="@string/fa_close_window"