Add support for additional navigation sounds

[ken-myers]

Issue description

Currently, the only theme-customizable navigation sounds are scroll sounds. I propose adding support for additional navigation sounds through a schema like this:

<navigationSounds>
    <scrollSound>./path/to/scrollSound.wav</scrollSound>
    <backSound>./path/to/backSound.wav</backSound>
    <selectSound>./path/to/selectSound.wav</selectSound>
</navigationSounds>

This block could be included in the top level of the theme and specified/overridden in narrower scopes like view, imagegrid, and textlist.

Happy to work on this myself and revise the schema or other details as necessary.

[fabricecaruso]

Then don't open an issue, but propose a PR ;-)