Add d3coder-functions to DevTools pages

[izzy]

From the Extension Support page:

I want to use this inside devtools so I can right click some items in the network section and decode them right on the spot instead of copying to another application.

Any chance you can manipulate the devtools context menu to include other than 'copy' the 'encode' function as well?

This could be possible through defining a devtools_page.