remove convenience.js and collect it from gnome.org instead

Created by: ghost

convinience.js is provided by the main gnome-shell project as boilerplate for extension developers. As there is no viable dependency management to speak of we can either ship a verbantim copy ourself as part of the repo or (as this PR introduces) manually fetch the authorative file straight from the gnome project when building the extension.

This introduces wget as a dependency for the collect make target.