Follow other GNOME apps designs by having fullscreen button on the right side of window header bar. The control panel had too many buttons already and we still need to make some space for playlist. This way "fullscreen" button will be on top bar while windowed and "unfullscreen" button will appear on the bottom right only when player entered fullscreen mode.
Create volume button with popover instead of using GTK provided volume button.
Creating new button with only needed elements is more efficient then removing unneeded items from pre-made volume button. This should also increase performance a little when changing volume, because now we generate new icon only when a change is needed. In pre-made button icon is regenerated on each volume change.
Seeking during slider drag is very CPU and HDD intensive task. We are requesting the player to keep seeking in VERY short amounts of time. This can be performed more effectively by doing a single seek after slider drop. Since this is a different behaviour then usually in media players, I am making this optional (enabled by default).
