UPDATED: Multi-Item Edit v4.0
Multi-Item Edit v4.0 allows you to edit most tags (and some additional options) of the selected track(s) in a single always-available 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.
This version adds Save and Load options so that a configuration of tag and options settings can be stored for later recall and use; adds Information Tracking toggling; adds a "Clear" feature; minor performance fixes.
Multi-Item Edit is free to use full-featured for ten days, $1.99 thereafter. More info and download is here.
Check out the video demo: