increment is too high on scroll over the volume slider

when you mouse scroll over the systray icon, the volume is changed by 5% step. It's hard coded in xlgui/tray.py with self.VOLUME_STEP = 5

whereas when you mouse scroll over the volume slider in the main gui, the step is 20%. It is way TOO MUCH.
it's because in exaile/xlgu/main.py there is incr = self.volume_slider.get_adjustment().page_increment

that could be changed to incr = self.volume_slider.get_adjustment().step_increment

how do you normally do a page increment with the mouse ?

but even with a step increment, it's hard coded in data/ui/main.ui at 10%

so. more consistancy will not hurt, 5% everywhere ?. and please remove the 20%, it could have hurt my ears ;-)