Since especially Windows puts the eject functionality behind an icon in the
systray which is usually hidden and doesn't complain if a USB drive is
unplugged without ejecting it first ejecting such a device might not be
obvious to everyone. Add a button to the main window allowing to eject the
selected player.
Currently only implemented for Windows.
Change-Id: I785ac1482cda03a1379cf6d0fd0d9a0ff8130092
