Refactor FileSearchQuery and create base class SearchQuery

Summary: As discussed in  [[ https://phabricator.kde.org/T10521#177460 | T10521 ]], it is useful to refactor out the generic query building logic in FileSearchQuery and create the base class SearchQuery. This would allow us to create other domain specific queries without having to reimplement all the building logic, as will be the case with TeamdriveSearchQuery.

Test Plan: All tests defined in FileSearchQueryTest should remain valid and, as of this patch, passing.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim, #libkgapi

Tags: #kde_pim, #libkgapi

Maniphest Tasks: T10521

Differential Revision: https://phabricator.kde.org/D19490