Use `chrome://` URIs for `require()` calls

Use `chrome://` URIs for `require()` calls

Apparently, `chrome://` URIs registered in chrome.manifest cannot be used in
bootstrap.js until its `startup()` function is called. Therefore this commit
reorganizes the code of bootstrap.coffee a bit, so that actual work is done
inside `startup()` (where it is supposed to be, after all). Outside of that
function we now mostly simply declare variables that the `shutdown()` function
needs to access as well. Some things were moved into main.coffee as well.

- Using `chrome://` URIs is the way add-ons are "supposed" to do it.
- If a `require()` call fails, we now get much shorter URIs in the error
  message, making things easier to debug.
- This allows to get rid of the hack of sending `__SCRIPT_URI_SPEC__` to frame
  scripts.