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...
fbe379c81c