Add 'download' icon

[mubes]

When a captured data set is already available on the scope, it would be useful to be able to pull that back into the scope without having to execute a new trigger. This is particularly important on slow scopes with deep data memory as it's much easier to configure a capture on the scope than on the UI.

[azonenberg]

Moved to scopehal-apps as this is a UI issue.

This should be as simple as adding a new toolbar icon (we need a sketch of what it should look like so I can go to the artist who did the other icons) that causes the scope thread to pretend a trigger event was emitted even when it wasn't. AcquireData() should work the same way regardless of how recent the trigger was, so none of that other code will have to be changed.