Log Message:
Update xfce4-diskperf-plugin to 2.6.1:

2.6.1 (2017/02/25):
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* stop always showing activity (#12942)
* fix sizing issues and GtkCssProvider lifecycle
* fix linux for devices with major > 255 (#13281)

2.6.0 (2016/11/1):
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* set progressbar minimal width/height to 4px