Fix parentheses for complex OR boolean search expressions

[Alkarex]

Search expressions combining some non-uniform parentheses and OR expressions where failing.
For instance:

• (ab) OR cd OR ef OR (gh) was wrongly parsed as (ab) (cd OR ef) OR (gh)

Improved the parser, also for expressions involving negations, and add several unit tests.

[Frenzie]

Could be an idea to add a couple of bad testcases to ensure predictable failure behavior? (E.g., mismatched parentheses.)

[Alkarex]

Could be an idea to add a couple of bad testcases to ensure predictable failure behavior? (E.g., mismatched parentheses.)

Indeed. At the moment, the parser never fails (unfortunately) but that is clearly something to improve, by failing more visibly when there is for instance mismatched parentheses

[Alkarex]

Also fixed ab OR !(cd OR ef), which was parsed as ab !(cd OR ef)