Calendar: Implement saving, loading, and displaying of calendars

The user can now save, load, and view calendars. A calendar is made up
of an array of events which are saved in a JSON file. In the future we
should implement the iCalendar standard instead of using a custom
format.

Userland/Applications/Calendar/EventCalendar.cpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2023, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "EventCalendar.h"
+#include <LibGUI/Painter.h>
+#include <LibGfx/Font/FontDatabase.h>
+#include <LibGfx/Palette.h>
+
+REGISTER_WIDGET(::Calendar, EventCalendar);
+
+namespace Calendar {
+
+static constexpr int tile_breakpoint = 50;
+
+EventCalendar::EventCalendar(Core::DateTime date_time, Mode mode)
+    : Calendar(date_time, mode)
+    , m_event_manager(EventManager::create())
+{
+}
+
+void EventCalendar::paint_tile(GUI::Painter& painter, GUI::Calendar::Tile& tile, Gfx::IntRect& tile_rect, int x_offset, int y_offset, int day_offset)
+{
+    Calendar::paint_tile(painter, tile, tile_rect, x_offset, y_offset, day_offset);
+
+    auto events = m_event_manager->events();
+
+    if (tile.width > tile_breakpoint && tile.height > tile_breakpoint) {
+        auto index = 0;
+        auto font_height = font().x_height();
+        events.for_each([&](JsonValue const& value) {
+            auto const& event = value.as_object();
+
+            if (!event.has("start_date"sv) || !event.has("summary"sv))
+                return;
+
+            auto start_date = event.get("start_date"sv).value().to_deprecated_string();
+            auto summary = event.get("summary"sv).value().to_deprecated_string();
+
+            if (start_date == DeprecatedString::formatted("{}-{:0>2d}-{:0>2d}", tile.year, tile.month, tile.day)) {
+
+                auto text_rect = tile.rect.translated(4, 4 + (font_height + 8) * ++index);
+
+                painter.draw_text(text_rect, summary, Gfx::FontDatabase::default_font(), Gfx::TextAlignment::TopLeft, palette().base_text());
+            }
+        });
+    }
+}
+
+}