TH4 Playlist validation (groups & tags)

[drikusroor]

This pull request adds Toontje Hoger 4 Absolute playlist group validation.

It checks if the group values in the sections are sequential and have the same length as the amount of sections. So let's say we have 6 sections, the (unique) group values should then be 1, 2, 3, 4, 5, 6. - Question to the reviewer: is that how the group values are supposed to be?

It also checks if the tags don't have values other than "a", "b" or "c".

Additionally, it moves the matching pairs test to the tests folder.

Resolves #1075 and #1091

[BeritJanssen]

The test includes irrelevant setUpTestData code, and does not test a scenario with valid tags.

The validation code itself seems problematic - for the groups, because they are strings, and will be listed in alphabetic order, for the tags, because distinct() will not deduplicate this way.