Updating the filtering methods used for searching.

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• Changing "filter by" methods so that each search class (user search, event search, equipment search) has exactly one filter method with optional parameters that covers all "filter by" methods being used before this change.
• Updating the use case diagram according to these changes.
• Updating "searching equipment" and "searching user" sequence diagrams in accordance with these changes.

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Added a Filter class with field and value variables, that has a 1-to-many relationship with Search Handler class.
Added search and sort method to Search Handler and thus unified the many search and sort methods in User Search, Equipment Search and Event Search classes.
Added RequestResult() method to each of the three child search classes.

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Added a sort action to search action in use case diagram.

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Updated search-related parts of "User searches for an event, then applies to it as spectator", "User searches for an event then applies to it", "User searches for a user" and "User searches for an equipment" sequence diagrams.