Make normal volume step smaller (Artful)

Issue:

When you increase or decrease volume using mouse (or by keyboard), it either increases too much or decreases too low. In other words volume step is too high. It varies among hardware because sound card doesn't controls the whole chain such that the max db information is correct

Gnome provided keyboard based solution where you have to press shift+volume-key to make it work. But it has several problem:

    a) On many laptops (Asus EEBook) and AIOs (Dell AIO) there is no separate volume-key. It is aggregated with function keys which doesn't work because third level and 5th level keys are broken. The key combination which are supposed to work are

                                        Fn + [F11/F12],
                                        Fn + +[Alt] + [F11/F12],
                                        Fn + +[Alt] + [win] + [F11/F12],

It varies with hardware and very confusing. Because pressing [Fn] + [Shift] simply doesn't work.

   b) On some modern touch screen notebook there is no regular volume key. There are keys attached to the screen but those keys doesn't work with "shift".

Patch:

This is a very small patch which simply modify volume_step and can be easily maintained. This also makes mouse-wheel behavior synchronous with keyboard. Thanks.