Add multi-to-one settings upgrading, and merge all the post-2.4 settings changes into __setting_upgrade_2__

Add this to your merge proposal:
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lp:~raoul-snyman/openlp/settings-upgrade (revision 2791)
https://ci.openlp.io/job/Branch-01-Pull/2295/                          [SUCCESS]
https://ci.openlp.io/job/Branch-02-Functional-Tests/2196/              [SUCCESS]
https://ci.openlp.io/job/Branch-03-Interface-Tests/2074...