Improve accessibility of the tempo and volume in play panel

A couple of remarks by our blind tester:

* The volume slider return very high number via the screen reader which are not helpful. To get there use the tabs.
* There is no possibility to change the master volume with the keyboard
* When using Tab to go the tempo input field, the screen reader doesn't state that it's for Tempo. Only when you tab next, it would read that it's the Tempo or Volume slider.