Remove grid_system on uninstall.

We already had 'delete', but that is deprecated, but according to the comment still needed on Plone 4.2.
Set up second grid and use it in vocab testing.
Test layoutmanager_settings with two grids.
Rename test grid system to Bootstrap3.
Testing: rename TwoGrids to MultipleGrids.

We might add a third grid for testing or for real at some point.