Minor changes and string fix in Mouse capplet

[Luca Ferretti <elle uca libero it>]

I've just committed a change to Mouse capplet with:

* Motion Threshold: --> Motion threshold: (as per HIG this should be
sentence, not header);

* Proper GtkSizeGroup for scales in General and Accessibility tabs;

* Fix indentation, spacing, border, padding here and there.

Cheers, Luca

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