xsettings: Disable middle-click paste by default
The middle-click will be used to start selections, and provide text contextual menus (such as word definitions, sharing, etc.) See: https://wiki.gnome.org/GnomeOS/Design/Whiteboards/Selections for design details. https://bugzilla.gnome.org/show_bug.cgi?id=665193
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f927057f