[bisq-network/bisq] Refactor FilterWindow (#3364)

[Łukasz Usarz]

@ripcurlx I'm trying to do small refactorings which eliminates code repetition, according to the `Leave your code better than you found it.` rule. They are rather easy to review - I'm explaining everything in pull request description. 

Doing such a small refactorings help me to familiarize myself with the code.

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