This switches internal storage from an (signed) int to an quintptr,
making it possible for client code to use Ids directly based on
pointers using Id::fromUniqueIdentifier().

Ids generated by existing code will keep using the low 32 bits only
for now, so the change effectively only affects 64 bit systems.

The patch also moves the rarely used fromUniqueIdentifier code
path out-of-line and replaces the Id::IdsPerPlugin and
Id::ReservedPlugins enums in the only place where they have been
used (setting a lower limit for ids). The limit itself not really
useful, but unchanged by this patch.

Change-Id: Ieabc7d9d72f3c5dd749fb49b7193a0c8629c186e
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>