Library merge fixes

[morski]

• Refactored the MergeVersionsManager
• Fixed issue Not merging tv shows/movies half of the time #19
  • Removed the filter where only items without PrimaryVersion are merged. The MergeVersions(List<Guid> ids) function can handle items that already are linked
• Fixed issue Deleting one version doesn't update the version count badge #48
  • The linked alternatives for the primary version are filtered so only the ones passed as parameters to the merge function are used

[morski]

Thanks for a great plugin! Was working on creating a similar fix for the 10.9 issue myself and found solutions to the problems linked in the description. Cleaned up the code a bit at the same time.

[danieladov]

Thank you for the PR!