create abstract Footer class

[mikemimik]

All the views will eventually be widgets which will take a footer class derived from this abstract or super class.

[mikemimik]

This is a non trivial task. In order to create a footer that is always at the bottom, and the content runs up until the height of the footer, you need to manage layout. The Scaffold widget is designed to do this. However using a Scaffold widget is also a non-trivial rewrite of how the calendar is fundamentally displayed.