mouse: Add support for enabling and disabling mouse acceleration.

Currently as described in #94 (closed), in gnome the setting enabling or disabling mouse acceleration, requires the installation of an extra utility like gnome-tweak-tool, or messing around in the command line. This feature is super useful for a wide range of people who dislike mouse acceleration such as many gamers, for whom the ability to disable mouse acceleration is almost essential.

This fixes this issue in a intuitive way, by treating 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 fixes: #94 (closed)

Side note: This is my first time trying to contribute something like this, and I almost certainly did something wrong, so feedback is most appreciated!