This commit hooks up AudioDevicesOptions to the new events exposed by
the AudioManager of osu-framework. The device list is now updated when
new devices become available or are lost.
Changed the name of the option for using the default audio device
to "Default" for consistency with english windows. Also changed
the "" value to string.Empty for consistency with the Frameworks
treatment of this special value.
The existing OptionDropdown only supported enums and was thus renamed
to OptionEnumDropDown. A new OptionDropdown has been created in its
place to allow binding to arbitrary values, with a set of user-provided
items.
