qml: reset progress bar mouse interaction on current item change

• Reset the mouse interaction so that a new press needs to be initiated when the current item changes. Currently, it is problematic if you slide to the end, as the player would instantly skip many songs because the position is not changed on mouse release.
• Do not position the pointing tool tip over the natural boundaries of the slider.
• Use mouse area width instead of control width in progress bar. Is not this the correct way of normalizing the value?

Note that this might not work as expected because the UI and the player are not synchronized. By the time the UI is informed about current item change, it might have already sent a request of position change.