API-cleanup: Add QPlatformFontDatabase::registerAliasToFontFamily().

Unexport free function qt_registerAliasToFontFamily() and
Make it a static member of QPlatformFontDatabase instead.

Change-Id: I1df49a8e37a24b3961f92288d67b6f1108a7d520
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>