Lays out elements in a left-to-right, top-to-bottom arrangement. The elements
on a given row will be top-aligned with each other. This layout will not work
well with elements that have a percentage-based width (e.g.
plotOutput() at its default setting of
width = "100%").
Unnamed arguments will become child elements of the layout. Named arguments will become HTML attributes on the outermost tag.
Any additional attributes that should be used for each cell of the layout.