Refactors the Calendar widget into LibGUI and updates the Calendar
app interface. Calendar widget lets layout engine manage most of
its geometry now and has a few new features like tile click
navigation, hover highlighting and a togglable year/month mode.
During app teardown, the Application object may be destroyed before
something else, and so having Application::the() return a reference was
obscuring the truth about its lifetime.
This patch makes the API more honest by returning a pointer. While
this makes call sites look a bit more sketchy, do note that the global
Application pointer only becomes null during app teardown.
This allows us to construct menus in a more natural way:
auto& file_menu = menubar->add_menu("File");
file_menu.add_action(...);
Instead of the old way:
auto file_menu = GUI::Menu::construct();
file_menu->add_action(...);
menubar->add_menu(file_menu);
