Lazy loading

[sproctor]

I'm going to use this issue to form a bit of a strategy on implementing lazy loading.

There are a few different aspects that can improve performance here:

1. Avoid loading rows that are off the screen. This is a bit problematic as it means that we can't measure the cells. My idea is to have column widths that can be implemented without measurement and a flag passed to Table or different function to enable lazy loading and disable measuring every cell that will raise an exception if a column width that requires measuring is used.

2. Avoid rendering off-screen rows. Simple, can be done ASAP, not sure how much benefit it will be.

3. Pagination. Requires an expansion of the API. Every cell of the page could be measured relatively cheaply. If columns can change widths between pages.