LibGUI+Calendar: Make Calendar a common widget in LibGUI

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.

Libraries/LibGUI/CMakeLists.txt

@@ -8,6 +8,7 @@ set(SOURCES
     Application.cpp
     BoxLayout.cpp
     Button.cpp
+    Calendar.cpp
     CheckBox.cpp
     Clipboard.cpp
     ColorInput.cpp