External url support when building urls

Currently we have special "external:" keyword witch can be used in "kApplication::Redirect" method only. When target template name for that method is prefixed with that keyword, then it's assumed, that full url to other website is given and redirect should made to it ("external:http://other-web-site.com/some_page.html").

I propose to move such keyword processing directly to "kApplication::HREF" method, which is called later from "kApplication::Redirect" method. This will allow us to build links to other sites using "m_Link" tag and in particular will help to simplify code of "st_CachedMenu" tag.