UPDATED: Multi-Item Edit
Multi-Item Edit v5.0 has been updated to accommodate the new Dislike, Work and Movement tags that are available in iTunes 12.5, currently in beta.
Multi-Item Edit will allow you to edit most tags (and some additional options) of the selected track(s) in a single floating window using single-edit mode (one selected track) or multi-edit mode, which emulates the pre-iTunes 12 "multiple items" format; that is, a checkbox adjacent to each tag allows you to select which changes are to be applied to the selection's tags.
As I mentioned above, this latest version adds new tags that will be available in iTunes 12.5; makes UI adjustments to accommodate those additions; removes the "Played" option (which tapped the AppleScript unplayed track property and which I'm not certain still performs a pertinent function anymore); includes minor maintenance and security fixes.
Multi-Item Edit is free to use for ten days and costs $1.99 thereafter.