Place this (as a file, called MyToolbarButton.addin) into the \accessory\addins\ folder (or a sub directory) and you will get a new button on the IDE toolbar that will launch an executable.
This is the “No Code” method and is kind of cheating since you are abusing the “MenuItem” type to create a toolbar button but it works well enough 
Of course you cannot declare a tooltip but on the other hand, a ToolbarItem does not support the “link” attribute which is why you would likely have to call a custom class command.
Salt to taste!
If you want to dig further, here is the MenuItem doozer responsible for building menu items from addins:
and similarly for ToolbarItem: