Update slot signatures to comply with PyQt5.6's stricter checks.
Add this to your merge proposal:
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lp:~raoul-snyman/openlp/stricter-slot-signatures-2.4 (revision 2649)
[SUCCESS] https://ci.openlp.io/job/Branch-01-Pull/1722/
[SUCCESS] https://ci.openlp.io/job/Branch-02-Functional-Tests/1633/
[SUCCESS] https://ci.openlp.io/job/Branch-03-Interface-Tests/1571/
[SUCCESS] https://ci.openlp.io/job/Branch-04a-Windows_Functional_Tests/1332/
[SUCCESS] https://ci.openlp....