edit-util: introduce EditFileContext

edit-util: introduce EditFileContext

This is a rather large change which moves
the add and install logic into edit-util.

We store an EditFile array and the number of
elements, along with the edit markers used in
temporary files and whether to remove the parent
directories of the target files if they're empty
in an EditFileContext object.

Call edit_files_add() to add an file to edit,
and do_edit_files_and_install() to do the actual
editing (through create_edit_temp_file(),
run_editor() and trim_edit_markers()).
After that, edit_file_context_done() can be used
to destroy the object.