Fix unlinked sockets with the same gems importing as one socket group

[mcagnion]

Description of the problem being solved:

When an imported item has unlinked sockets holding the same gems, a single socket group is created for all of them. For example, a wand with Raise Spectre in each of its three unlinked sockets is imported as one Raise Spectre group, so two of the three spectres are missing from the build.

ImportSocketedItems compares each group of linked sockets on the item with the socket groups already in the build, so that a reimport without "Delete skills" refreshes an existing group instead of adding a copy. The socket groups already added for the same item were compared too: every unlinked socket after the first matched the first socket's group, so it only refreshed that group's gem levels and quality instead of adding a group.

A socket group that was matched or added for the current item can no longer be matched, so each group of linked sockets gets its own socket group. This applies to a fresh import and to a reimport with or without "Delete skills". A build that was imported with one merged group gets the missing groups back on a reimport without "Delete skills", and the existing group is kept as it was, apart from its gem levels and quality.

Steps taken to verify a working solution:

• New test: with one Raise Spectre group already in the build, reimporting without deleting skills an item with Raise Spectre in three unlinked sockets gives three groups. It fails without this change (one group).
• Imported a real character whose off-hand wand has Raise Spectre in three unlinked sockets: one group before, three after. Its main-hand wand (two linked Raise Spectre plus one unlinked) and its gloves import as before.

Before screenshot:

After screenshot: