[BUGFIX] Fix custom thumbnails bug and add events

Classes/Controller/YoutubeController.php

Classes/Event/AfterFilterVideosEvent.php

Classes/Event/AfterMapCustomThumbnailsEvent.php

Classes/Event/AfterYoutubeCallEvent.php

Classes/Event/BeforeYoutubeCallEvent.php

Classes/EventListeners/AfterFilterVideosEventListener.php

.../EventListeners/AfterMapCustomThumbnailsEventListener.php

Classes/EventListeners/AfterYoutubeCallEventListener.php

Classes/EventListeners/BeforeYoutubeCallEventListener.php

Classes/Upgrades/ThumbnailsUpgradeWizard.php

Configuration/FlexForms/flexform_sgyoutube_youtube.xml

Configuration/Services.php

Event/PreYoutubeApiCallEvent.php

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