Interface generator - add support for "write" type properties

Those properties will have the setter code generated but have
difference compared to usual "readwrite" properties. sd-bus
does not look like supporting "write" properties anyway.

Also fix trying to access the `is_read_only` attribute on unknown
property access type. Use actual `access_type` variable to raise
exception.

src/sdbus/interface_generator.py

@@ -436,12 +436,12 @@ def __init__(self, element: Element):
         self.is_explicit = False
 
         access_type = element.attrib['access']
-        if access_type == 'readwrite':
+        if access_type == 'readwrite' or access_type == 'write':
             self.is_read_only = False
         elif access_type == 'read':
             self.is_read_only = True
         else:
-            raise ValueError(f"Unknown property access {self.is_read_only}")
+            raise ValueError(f"Unknown property access {access_type}")
 
         super().__init__(element)