TriblerButtonsDelegate is repainting all table rows on mouse movement

[devos50]

See here. There already is a TODO in the code related to this but given that our table rows are getting more complicated with the addition of tags, it can get quite slow. As such, I increase the visibility of this enhancement with an issue.

This should be relatively easy to fix since the only rows changing on mouse hover is the row referenced by self.hover_index and the newly selected row.