Add new public APIs to find the z-order for links and widgets.

- Implement FPDFLink_GetLinkZOrderAtPoint().
- Implement FPDFPage_FormFieldZOrderAtPoint().
- Mark FPDPage_HasFormFieldAtPoint() as deprecated.
- Modify CPDF_LinkList and CPDF_InterForm to support new APIs.
- Clean up dead code in CPDF_LinkList and CPDF_InterForm.

BUG=chromium:515837
R=jun_fang@foxitsoftware.com, tsepez@chromium.org

Review URL: https://codereview.chromium.org/1278053004 .