Use QVector instead of QList for the list of active effects

We only append to the list, so the advantage of insertion into QList
does not matter.

Thanks to Milian Wolff to point this out in his Akademy talk [1] and for
pointing me to Marc Mutz blog post [2] discussing this topic and
recommending the usage of QVector over QList.

[1] http://milianw.de/files/apps-on-speed-akademy2013/index.html
[2] http://marcmutz.wordpress.com/effective-qt/containers/

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