Context menu shows divider when items are hidden

[TomTirapani]

[TomTirapani]

Pull request for fix here: #519

We already had code in place to filter out consecutive dividers and filter out starting / ending dividers. However, both filtering steps were happening in the same pass over the array. This meant if there were consecutive ending dividers, one would remain.

See the defaultContextMenuItems here: https://github.com/exhi/hoist-react/blob/develop/desktop/cmp/grid/GridModel.js#L40. You can see how a grid with neither export nor column chooser would end up with 2 consecutive ending dividers.

In the pull request, I split these filters into 2 different passes. Now starting / ending dividers are filtered out after any other mutations.