LibGUI: Add Calendar property for mode

If a user selects Year as the default view mode for Calendar app, then
all apps using Calendar widget will default to this view if not
manually overridden with a Calendar::toggle_mode call.

This commit introduces a "mode" property that allows the selection of
the default mode for the calendar widget in GML files. In this way
there is no need to manually call toggle_mode when constructing
UIs with Calendar widget.

Userland/Libraries/LibGUI/DatePickerDialog.gml

@@ -30,6 +30,7 @@
 
         @GUI::Calendar {
             name: "calendar_view"
+            mode: "Month"
         }
     }