Bug 707112 - Replace webkit_dom_html_element_get_id usage

webkit_dom_element_get_id() was introduced for WebKitGTK+ 2.2 and
deprecates webkit_dom_html_element_get_id().  But Evolution only
requires WebKitGTK+ 2.0.1.

Ordinarily this would sit on a shelf until we require WebKitGTK+ 2.2,
however WebKitGTK+ has now started issuing deprecation warnings about
webkit_dom_html_element_get_id() at *runtime*, which is pointless and
only fills up users' .xsession-errors file.

To put a stop to this, we'll call the new function subject to a pre-
processor WebKit version check, with a fallback to the old function.

The build requirement remains at WebKitGTK+ 2.0.1.