Support QCborMap::operator[] taking a string literal

Use a template on the size of the char[], as suggested by Ville
Voutilainen.  This resolves ambiguity about whether such look-ups
should be done via QString or QCborValue (not that it would have made
any difference).

When we come to add mutating indexing of QCborValue, chained
dereferences like map[i][j][k] need to stay in operator[] const
throughout, to avoid detaching intermediates to create references into
them due to using the mutating operator[] on the earlier dereference's
return.  So const-qualify the QCborValue operator[] const variants at
the same time, to match those of QCborValue itself.

Change-Id: Ib1652ae9440fe3767a653afa2856b74040210e07
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>