Support visible=no for main page tab.

Not sure why this was hard-coded in there, but I am using Doxygen to generate parts of a larger set of documentation, and the "main page" isn't Doxygen created (and the main Doxygen page is useless in this case).

I am not the only one who's had this issue: http://stackoverflow.com/questions/21182270/doxygenlayout-mainpage-visible-no-does-not-work

I think Doxygen should just do what the layout file says.  Alternatively, being able to specify a custom URL for the mainpage tab would also do.