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.cpp

@@ -64,6 +64,10 @@ Calendar::Calendar(Core::DateTime date_time, Mode mode)
     }
 
     update_tiles(m_selected_date.year(), m_selected_date.month());
+
+    REGISTER_ENUM_PROPERTY("mode", this->mode, this->set_mode, Calendar::Mode,
+        { Calendar::Mode::Month, "Month" },
+        { Calendar::Mode::Year, "Year" });
 }
 
 void Calendar::set_grid(bool show)
@@ -85,6 +89,13 @@ void Calendar::toggle_mode()
     invalidate_layout();
 }
 
+void Calendar::set_mode(Mode mode)
+{
+    if (mode != m_mode) {
+        toggle_mode();
+    }
+}
+
 void Calendar::show_previous_date()
 {
     unsigned view_month = m_view_month;

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);

Userland/Libraries/LibGUI/DatePickerDialog.gml

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