messageTray: Considerably rework how layout is done in notifications

Use a series of nested BoxLayouts, Bins, and more instead of an StTable
and a custom ShellGenericContainer, and hacky style classes.

This also removes the customContent parameter in favor of subclasses
setting the child of this._bodyBin instead.

We lose a few of the fancy features like showing the first part of
the body, ellipsized, next the banner when it will fit, and other
layout logic. But since the layout of notifications is changing
substantially anyways, I don't feel too bad...
messageTray: Restyle notifications
messageTray: Glue the notification to the bottom of the screen, always

We'll animate notifications popping up with another system soon enough,
instead. The idea here is that instead of carefully animating the Y
position of the notificationWidget when a notification updates, we
simply animate the height of the new actor inside the notification.
This will fix some of the awkward updates where instead of the
notification content expanding, we see the buttons or action area
pushed off the edge of the screen...

Animations that happen as a result of adding something new to the
notification or expanding it should be done by tweeing the new actors
in inside the notification.
messageTray: Add a Revealer widget and use it to animate the notification in/out
messageTray: Use a revealer when showing notification bodies as well
telepathyClient: Use a revealer for new messages in Telepathy as well