introduce panes

[naw]

@jgautsch

Here is a first stab at adding panes. I think there are several ways it could be improved.

However, the first thing to improve is that it isn't actually memoizing the matchingVisibleTodos for each pane individually --- so if you change the search term in one pane, it recalculates for all panes.

I'm not immediately sure how to build a reselect selector to address this issue. Any ideas?

[naw]

I think this pane PR ties in well with the discussion about global selectors vs. memoized component calculations.

Do you know of a way to calculate and memoize the derived data for each pane using a global selector? To be clear, if one pane changes its searchTerm, that should recalculate and memoize the visibleTodos and matchingTodos for that pane, but not recalculate and memoize for the other panes. Currently this branch is recalculating for all panes whenever a single pane changes.

[jgautsch]

For posterity: This is the good one.