Use strings instead of functions in the `modes` object

Use strings instead of functions in the `modes` object

Since we run things properly in the `startup()` function of bootstrap.js since
last commit, the `chrome://` URI to the string bundles is now available when the
`modes` object is constructed. This means that there's no need anymore to let
mode names, category names and command descriptions be functions. Instead they
can be the sane thing: Simply strings.

Note that this is a backwards incompatible change. Luckily it does not affect
the `.addCommand()` API method, so propably won't impact anyone.