Seperate GUI & Actions for SDK

Pretty much every application consists of a set of basic commands. For
instance, in Photoshop you would have "AddLayer", "SelectRectRegion",
"InvertColors", etc. Each of these commands could be linked to a menu
item, a button, etc. What I need to do is seperating these commands
from the GUI so that I can offer them in an SDK. The user then can
create his own GUI element and associate it with one or more such
commands. Equally important, he could use it to build scripts to
automate processes. The question is how do I best do this? How should
these kind of commands look like? Any input is very much appreciated.

Regards