Define a macro for generating the index page title with the correct
Qt version (major.minor).

This way, the version is automatically updated on the landing page,
based on .qmake.conf.

Change-Id: I44fcc76bfe4ac899a4d3edb334892b088e021d56
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>