Create URL form element

In order to fix #103680: Path alias validation should test for full URLs I want to add an URL element that allows developers to create text fields that accept absolute URLs, URL paths or URL aliases or a combination of these three.

What this patch should fix, directly or indirectly:
- Automatically removed leading and trailing slashes from URL paths and URL aliases (UX problem and bug).
- Centralised validation for different kinds of URLs, which makes code easier and fixes numerous bugs.
- One element that can accept different kind of URLs. It is possible with textfields, but that's not as clean.

For this menu_valid_path() needs to be split in a function that checks if a URL path is actually valid (like the name suggests) and a function that does an access check. Path_admin_form_validate() and path_form_element_validate() also need to be changed to provide one URL alias validation function. Both new validation functions should simply return a boolean.

This approach would fix numerous issues across Drupal. It does not really change the API, but it rather extends it.