compiler: private properties in an interface are an error

It does not make sense to declare an interface with properties that
are not accessible to the user of the interface.

Author: 0x6e

internal/compiler/object_tree.rs

@@ -1281,7 +1281,7 @@ impl Element {
             });
 
             if base_type == ElementType::Interface && visibility == PropertyVisibility::Private {
-                diag.push_warning(
+                diag.push_error(
                     "'private' properties are inaccessible in an interface".into(),
                     &prop_decl,
                 );

internal/compiler/tests/syntax/interfaces/interface_properties.slint

@@ -3,10 +3,10 @@
 
 export interface ValidInterfacePropertyDeclarations {
     property <bool> implicit-private;
-//  ^warning{'private' properties are inaccessible in an interface}
+//  ^error{'private' properties are inaccessible in an interface}
 
     private property <int> private;
-//  ^warning{'private' properties are inaccessible in an interface}
+//  ^error{'private' properties are inaccessible in an interface}
 
     in property <int> in-property;
     out property <string> out-property;