LibGUI: Remove Tile.date_time member from Calendar

Currently, navigating through different years in the Year view of the
Calendar app or the taskbar is very slow.
Profiling results show that almost all the time is spent in
`Calendar::update_tiles`, and specifically, in `DateTime::create` and
`DateTime::set_time`.

Performance can improve substantially if the `TZ` environment variable
is set [0], but we can improve the current code to perform better
nevertheless :^)

This diff focuses on removing the need of the `Tile` struct to require
the instantiation of a `DateTime` object, which avoids _at least_ 365
object instantiations in the Year view, on each `update_tiles` call.
Instead, as the `date_time` isn't used often, we can instantiate it on
demand when a particular date is selected.

[0] https://blog.packagecloud.io/set-environment-variable-save-thousands-of-system-calls/

Userland/Libraries/LibGUI/Calendar.h

@@ -101,7 +101,9 @@ private:
     Vector<MonthTile> m_months;
 
     struct Tile {
-        Core::DateTime date_time;
+        unsigned year;
+        unsigned month;
+        unsigned day;
         Gfx::IntRect rect;
         int width { 0 };
         int height { 0 };