Save selected traffic

[mossmann]

Add ability to select multiple rows in traffic pane and save selection. Possibly also allow launch of external program with selected traffic data as input.

[martinling]

This is going to be a bit more work than I thought; I had expected we could just replace SingleSelection with MultiSelection, and then ask that for the row ranges to save, but the API to MultiSelection doesn't provide that information; you can only ask it whether a given row is selected, or tell it to give you a Bitset over all rows, which it warns is slow.

So I think we're going to want our own SelectionModel, which handles selection actions and then keeps track of what was selected in a form that allows us to efficiently iterate over just the packets to be saved. That doesn't look too hard to do, though.