perspective-viewer-hypergrid Tree column toggle buttons.

[texodus]

@finos/perspective-viewer-hypergrid now has context toggle buttons for tree grids, where previously the entire cell simply responded to clicks. This makes mixed selection/toggle work, as well as generally more sanitary UX.

Also some style tweaks/refactors are included:

• Hypergrid tree columns which are smaller than the viewer width are no longer hidden.
• When load() is called and view config attributes have already been set, reset() will now automatically be applied if the current view config triggers an error with the new table.
• Reduced hypergrid resize thrashing.
• Subtler colors for <select> controls.