Invalid filters

[Ro4052]

#349 UI filters should give user feedback when they are valid

• Exposed new table method for determining if a filter is valid
• Viewer validates filters before passing to table
• Invalid filters operands are underlined in red with a red exclamation mark next to them
• Filtering by an empty string is now invalid (this seems more appropriate for the current viewer UI)
• Added a screenshot test for the new invalid filter highlighting
• Added test for filtering by empty string
• Added tests for newly exposed is_valid_filter method

[texodus]

Thanks for the PR! This is a great addition - User experience is awesome, and the tests in particular are greatly appreciated!

I think as a future enhancement we may want to reconsider this API, in favor of general validation across APIs which take configuration objects as parameters - see also my comments on API differences between viewer and library. I think this will help keep the API surface small, predictable, as well as portable and consistent between host languages.

As this is currently undocumented, IMO it is fine as is for now.