This PR adds a "Page layout" setting to the Framed Theme, with a new single page layout that shows the menu on every page instead of in a frame.
It's meant for people who can't or don't want to use frames, so the Legacy Theme is no longer the only option without frames.
The menu code moved out of `left.cgi` into a shared menu-lib.pl, so both layouts build the same menu, frames look the same as before, and `left.cgi` shrinks from about 400 lines to about 60.
The menu's module search box now follows the global "webminsearch" permission, which the old frame code never actually checked.
One exception to be aware of. A server opened through Webmin Servers Index always uses frames, because that index only rewrites the links and redirects of framed pages.
