Add a Toontje Hoger rules playlist validation method

[drikusroor]

Using the new #978 rules playlist validation feature, it would be nice to have rules playlist validation for Toontje Hoger as well. I myself (Drikus) am not too aware of the specific requirements for the Toontje Hoger experiments' playlists, so I think it would be better if @BeritJanssen would specify those.

Requirements

• Toontje Hoger 1 Mozart Effect
  • TH1 - Sections need to have a group value of "1" or "2" #1069
    Added: Add validate_playlist method for Toontje Hoger 1 - Mozart experiment rules #1070
• Toontje Hoger 2 Preverbal
  • Toontjehoger 2 validation #1072
    Validate ToontjeHoger 2 (Preverbal) playlist #1079
  • TH2 - Sections need to have a group value of "1" or "2", i.e. ^[12]$ #1078
• TH3 - Toontje Hoger 3 Plink #1073
• Toontje Hoger 4 Absolute
  • TH4 Playlist Validation: Sections need to have a tag value of "a", "b" or "c", i.e. ^[a-c]$ #1091
  • TH4 - Sections need to have a group value of a number from 1 to PLAYLIST_ITEMS #1075
    TH4 Playlist validation (groups & tags) #1090
• Toontje Hoger 5 Tempo
  • TH5 - Sections need to have a tag value that matches the following regex #1076
    TH5 Playlist Validation: Add playlist tag validation to ToontjeHoger(Kids)5Tempo rules #1081
  • TH5 - Sections need to have a group value of "or" or "ch", i.e. ^(or|ch)$ #1077
    Added: Add playlist group validation to ToontjeHoger5Tempo rule #1080
• TH6 - Toontje Hoger 6 Relative #1074