Rockbox

This patch adds an action to the option_screen to cause the default value
for a particular setting to become the current selection. It allows you
to see which value is the default, and at that point you can confirm it
or not. The code to do this is all in there, it just needs to be moved to
separate functions so we can reuse it.

Particularly useful for settings with long ranges of options, like a lot
of the sound settings such as base and treble for example. I'm blind, and
I find it takes a lot of button'ing around to put the setting back to
zero. It's also great for when music is playing and the sound menus are
not spoken. And as a bonus, it lets one see what the safe default option
is supposed to be when browsing an obscure option.

Do others think this is useful?

The only tough part is the button assignment of course. This patch has
button mappings for x5 and e200 only, and at this point I've just added
them to the list context, but perhaps a separate context could be
created. For X5 I chose SELECT+POWER. (I thought the awkwardness wasn't
a big issue in this case.) For e200 it's POWER+SELECT.