Undo: Add hint about what is being undone

It would be nice that when placing the mouse cursor over the "undo" icon, the tip that pops up gave a hint about what operation is to be undone. The same would be useful when undoing from the edit menu.

This is a very common feature in many programs, particularly in graphic programs (they go as far as undoing selections), or audio editors (see for instance GIMP, Inkscape, Audacity, even LibreOffice), and is very handy when there are "invisible" or subtle changes that are difficult to remember, such as stretching slightly a measure, or applying a velocity offset to some note, or changing some individual note in a complex texture.