Deprecate QMap implicit conversion to a multi map

Functions that enable this behavior:

• insertMulti
• unite

Functions that are rendered redundant by this change:

• equal_range(Key)
• count(Key)
• uniqueKeys()
• values(Key)

These functions will also be deprecated and slated for removal in QMap. QMultiMap will retain these and unite() (insertMulti will be gone for both, QMultiMap::insert does the same).