Allow filtering samples by compound expressions including multiple scorers

[amatveiakin]

Allow filtering samples by compound expressions like result == "C" and steps <= 10.

• Expression parsing via filtrex library. Supports arithmetic, basic math functions, Python-style boolean operations, chained comparisons.
• The filter expression can include any scorer, not just the selected one.
• Clicking on a score adds it to the filter. Moreover, for simple categorical scores the UI will automatically suggest expressions like result == "C".

Auxiliary changes:

• Merged scorer and score selectors. Strictly speaking, this was not necessary to unblock this change. I thought it made sense because I made the filter field wider, so the tools panel is now more prone to overflowing. But we can revert if needed.
• Moved filter and scorers to the right so that they are nicely aligned.
• Made scorer list collision-proof. Now if two scorers define scores with the same name, the scorers panel will use dot notation to disambiguate, e.g. score.foo vs other_score.foo.
• Implementation detail: Split SamplesDescriptor into SamplesDescriptor (depends on the current scorer) and EvalDescriptor (does not depend on the scorer).

Next steps:

• This UI allows extending filter expressions without adding visual clutter. I would like to eventually allow filtering by sample metadata and full-text search over the transcript.
• Consider if the tool order could be improved. I find it a little confusing that the filter is to the right of the scorer selector, yet does not depend on it. Not sure how best to fix this, because I want to keep the filter aligned with the scorer list.

[dragonstyle]

Thank you for this! It's a holiday for the next couple of days in the USA so I'll really dig into this on Monday!