Allow file drag-and-drop

[fredemmott]

Why is this change being made?

Just a small ease-of-use improvement. Because of other dev tools, I mostly expected this to work.

Briefly summarize what changed

Executable and PDB files can be dragged into the 'Examine a binary' and 'Start a diff' windows.

I took a brief look at also supporting dragging on to the 'Welcome to SizeBench' pane, but not familiar enough with WPF to figure out a clean way to do that with the routed UI Commands.

How was the change tested?

Dragged my exe and PDB into the 'open single binary' controls, clicked OK, sizebench opened it up

Dragged them separately into before and after in the diff window, only the dropped control was popuplated.

PR Checklist

• Contributor License Agreement (CLA) has been signed. If not, go here and sign the CLA
• Changes have been validated
• Documentation updated. Please add or update any docs in the repo as necessary.