mouse: Added support for enabling and disabling mouse acceleration.

This treats mouse acceleration as a binary enabled/disabled, enabled being
the adaptive profile, and disabled being the flat profile, in gsettings
there is also a default value, however from what I can see this is simply
the adaptive profile by another name, so I treat it as such.

This feature is super useful for a wide range of people who dislike
mouse acceleration such as gamers, and as such a core setting it should
not rely on the user changing config settings to change.

This fixes: #94
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