WelcomePage: fixes for the filter combobox for examples

"All Versions" did not work correctly and just contributed to
confusion. So it is removed. Since we need a sensible default
setting we now search for the Qt with the highest version that has
examples and demos.

Task-number: QTCREATORBUG-9531
Change-Id: I495252a99753ebe511b598c61a79bbb0cd293e40
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>