Don't overwrite the sandbox console object with the Firebug console.

Recent versions of Firebug attach the console to the content window
as a passive console API listener. Thus, it automatically handles
calls to any console API function, without having to be manually
assigned to the window's console object.

See http://git.io/7DMnWA.

Fixes #1949.
Move GM_openInTab to miscapis.js.
Move extractMeta into its own file.
Remove the need for a chrome window reference in GM_xhr.
Refactor parts of the menu command handling into a JSM.
Move menu command event handling to GM_MenuCommander.
Move sandbox related functions into JSM.
Remove unused imports/definitions.