Search filter improvement: not operator, better parsing

[80avin]

Checklist

• [x] I've looked through the documentation and existing open issues for similar feature requests.

Describe the feature request

I wanted to run a search query "user != root" or "state != sleeping" but apparently "!=" or "!" operators aren't supported. Digging into the code to find where to add, I found more improvements are possible.

Better Filter syntax

The search can be made in 3 modes

• Literal mode: activated when user types a plain string without operators or quoted string ( with escapes ) Action: whatever is typed is matched against Name. Equivalent to double-quoting the text.
• Regex mode: activated when Regex(Alt+W) is set, OR when user wraps the query in /query/. Latter can support flags like i too. Action: whatever is typed is parsed as regex and matched against Name.
• Query mode: activated when Regex(Alt+W) is unset and user types an operator. Action: Executes query as a boolean expression, where:
  • Value can be literal ( "quoted", 'quoted' or numbers ), regexp: ( /value/ ) or more shorthands for time, bytes, percentages, etc.
  • operators != (not equal), ! (not), ~ (partial match) and current ones are supported. We can also support more operators like in or like operators from sql, but having regexp is enough I guess.

Looking for feedback on the design before I try an implementation. I think this will improve the parser too, making it a proper expression parser.