feature request: expose ability to disable stepping buttons

[danallan]

For a C debugger, we are directly manipulating the "disabled" attributes of some of the debugger's UI elements for a variety of reasons. In particular:

• Our debugger (GDB) does not support stepping out of the main() function (it errors rather than stopping execution of the program, as we might expect)
• During segfaults, the stepping functions (play/pause button, step over, step into, step out) need to be disabled because it doesn't make sense to allow any further execution.

In the latter case, it doesn't suffice to simply stop debugging and hide the debugger UI; we certainly want the ability to navigate the stack to observe the state of the program at the point of the segfault, but we just want to disable the ability for users to continue executing the program.

It would therefore be great to be able to provide API access to change the "disabled" attribute of each of these buttons independently!