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/Calendar.h

@@ -66,6 +66,7 @@ public:
     ErrorOr<String> formatted_date(Format format = LongMonthYear);
 
     Mode mode() const { return m_mode; }
+    void set_mode(Mode);
     void toggle_mode();
 
     void update_tiles(unsigned year, unsigned month);