Respect container padding, rework gap/spacing accordingly

[graynorton]

Description

As discussed here...

https://github.com/PolymerLabs/uni-virtualizer/issues/17#issuecomment-554053084

...we'll change the way that virtualizer handles padding and spacing.

Currently, CSS padding on the container is ignored and there's a spacing property on layouts that determines both "outer" spacing (distance between the container edge and the outermost columns & rows) and "inner" spacing (distance between items).

Instead, we'll respect padding on the container and allow that to determine outer spacing, and we'll replace the spacing property with a gap property, a la CSS Grid, to control inner spacing.

Acceptance Criteria

Implementation and samples updated per the description above.