add a button to invert the selected set

[smurp]

Add a swap button with an icon like ⇄ in the upper right corner of the taxon_picker which inverts the selection. In other words, it deselects all selected nodes and selects all the deselected nodes.

Implement this capability in treepicker.coffee and hide it behind an argument to the constructor. It should be off by default so the set and edge pickers don't display it.

Ahhh. This should work slightly differently. Instead of being a button "at the top level" it should be a button visible at every level within the picker hierarchy which swaps the selection at that level BUT should work on every level below when the level is collapsed. So to swap the selection of everything one would collapse Thing and then click the swap icon.

[smurp]

A possible UNICODE icon for this is ☯