Move the MainWidget ui into its own file
MainWindow: Remove 'using' line
make all pages invisible by default

Otherwise, they screw up the order when constructing the MainWidget, and
we do a show_all on the MainWindow anyways.
MainWidget: Use the stack's visible_child property

instead of the visible_child_name
Call switch_page directly on the MainWidget

Unfortunately, we can't just proxy it through the MainWindow.