* Fix#4552: Multiple quoted search terms not parsed correctly.
* Allow quotes to be escaped in quoted metatags.
* Allow spaces to be escaped in unquoted metatags.
* Allow the empty string to be used in metatags.
Examples:
* `source:""` and `source:''` (same as `source:none`)
* `source:foo\ bar\ baz` (same as `source:"foo bar baz"`)
* `source:"don't say \"lazy\""` (use \" to write a literal ")
* `source:'don\'t say "lazy"'` (use \' to write a literal ')
* `source:"C:\\Windows"` (use \\ to write a literal \)
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