Reserve/pre-make a limited number of 'user commands' for keybindings & icons

[boltex]

Reserve/pre-make a limited number of 'user commands' which will be assignable to a command or 'chunk of code' in Leo, akin to it's @button system, and let the user define them.

Also assign a keybind to it as they will be mapped to a couple of 'premade-generic' commands. (probably going to be named 'leo custom 1' ... through 10 or something like that!