Added padding

[codecat]

This adds:

• lay_set_paddings
• lay_set_paddings_ltrb
• lay_get_paddings
• lay_get_paddings_ltrb

This acts as the opposite of margin.

Allows us to avoid creating a secondary container item with a margin to achieve the same thing and do it all in 1 item.

This mirrors my C# implementation here: https://github.com/codecat/Nimble.Layout/pull/1