shortcut mechanism. The existing code doesn't use any deprecated
calls, and translating shortcut text _into_ Pango markup just sounds
too unpleasant to do if I don't actually have to. Not to mention
that the documentation for the Pango markup language doesn't tell me
how to distinguish a mnemonic underscore prefix from a literal
underscore in label text, but I know my current code can get that
right (the current config box talks about TCP_NODELAY and
SO_KEEPALIVE in widget labels that also have functioning shortcuts).
[originally from svn r7961]