main: Update state of previous/next when chapters info arrives

Bastien Nocera requested to merge wip/hadess/fix-button-mkv-chapters into master

This fixes the previous/next buttons not being sensitive when a single file with chapter data is read as the "hey, a chapter is available now" would come in later than we usually check.

The keyboard shortcuts for previous/next track already worked as whether a chapter marker is available is checked when the key has been pressed.

Closes: #215 (closed)

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