The current graphical frontend of radare is based on a simple Gtk+Vte+Cairo application running a radare application inside the virtual terminal (vte) and feeding it with the output of some scripts called 'grsc' stored in $prefix/share/radare/gradare/ in subdirectories per categories.
$ pwd
/usr/share/radare/gradare
$ ls
Config Debugger Disassembly Flags Hacks Movement Search Shell Visual
$ ls Debugger/
AttachOrLoad Continue Detach Registers Status StepOver Stop
Breakpoint ContinueUserCode Maps SetRegister Step StepUserCode
$ cat Debugger/StepOver
#!/bin/sh
echo S
TODO ...