Add ChromeVox panel and implement caption display functionality.

Add ChromeVox panel and implement caption display functionality.

This patch creates a new HTML background page in the extension that's
formatted with a menu button, a caption area for speech and braille, a
settings button, and a close button.

Currently only the caption part functions - it uses cross-window messages
to send either the current speech from TTS, or the current braille text
and cells if braille captions are enabled.

This patch does not contain the code to show the caption panel on the screen,
that will follow. You can test it in the meantime by manually opening up
the URL.

BUG=420795