Expose buttons as `interactive' commands

[nbfalcon]

I often found it useful to view a symbol in the manual, jump to its source etc.,
but navigating to the corresponding buttons was cumbersome. As such, expose them
as interactive commands, with mnemoic keybindings beginning with C-c:

• helpful-view-in-manual (C-c m)
• helpful-goto-source (C-c s)
• helpful-toggle-edebug (C-c e)
• helpful-toggle-tracing (C-c t)

---

[nbfalcon]

This PR should now cover all buttons. There is one question though: helpful--show-callees (now helpful--show-callees-1), enables helpful-mode on the output buffer. However, this enables all the command keybindings (toggle edebug, ...) which don't work because the buffer doesn't set helpful--sym etc. Do you think that function should active a different major-mode, or to rather set helpful--sym, etc so that the commands can work even in show-callees buffers?