up() places the next cell above the previous one, and left() places the
next cell to the left of the previous one. down() and right() are
aliases to row()/col(), for symmetry.
Also added are nextX() variants for function.
Previoysly, padding was added before adding a cell, which lead to
incorrect and unaligned cell placement in nested layouts.
Now padding is only added when at least one cell has been placed before.
Revert signature if layout:push(x,y, padx,pady) to layout:push(x,y)
Makes it possible to use the idiomatic layout:push(layout:row()) again.
(layout:row() returns x,y, w,h of last cell)
Remove layout:pos([x,y]) because it's not needed and potentially confusing.
