support for non-node paths

Problem/Motivation

domain_path can only create domain-specific aliases for nodes. This leaves out lots of things like views and panels.

Of particular interest is this module's ability to have a single path (like /news) refer to different pages on each domain. This is possible currently with nodes, but not with views.

Proposed resolution

Remove the dependence on entity_type and entity_id, and instead refer to the source / alias sets directly in the same way as core paths.

Remaining tasks

patch needed.

User interface changes

The additions to the node edit form would remain. We would ideally have additional domain-specific settings on admin/config/search/path/edit/%, with something like
function domain_path_form_path_admin_form_alter(&$form, &$form_state) {

API changes

A big change internally, but since domain_path doesn't offer an API to other modules the impact would be slight.