Total Commander

All modern mice have at least three buttons.

Probably should add on the toolbar buttons a new option and a new field that handles pressing the middle mouse button.
For example, buttons options now has a field Command: cm_CopyOtherpanel
I want to have more field below: CommandM: cm_RenMov

Left Click = Command
Middle Click = CommandM

If user work with keyboard on buttonsbars, that middlemouse click maybe processed by Shift+Enter or Ctrl+Enter...

Support for a buildin solution.

ButtonBar eXtended can do so, but unfortunately its a bad workaround, that causes some problems here and there. (Keyboard modifiers work fine, only the mouse buttons are a little bit problematic)