Add ability to push interesting results onto a stack for future exploration

[aeisenberg]

Typically, when iterating through queries, a security researcher will find a handful of interesting results. This issue is about enabling the user to save specific results so that they can be explored in more detail later.

These results should be available cross-query and cross-database. I.e. there should be a single set of saved results for the entire workspace.

A few open questions:

1. Can we save path queries? How would that work?
2. Should we be able to store the stack across restarts?
3. What happens if databases are removed?