Batch Queue Manager : add new option to turn on/off multi-core support. This...

Batch Queue Manager : add new option to turn on/off multi-core support. This option is now turn off by default due to some dysfunctions under Windows, until it will be hack and fixed.
Note that problem are not reproducible under OSX and Linux here. Options is implemnted for digiKam 3.3.0 release.
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