qml: dynamically populate the toolbar editor

qml: dynamically populate the toolbar editor

Thanks to MOC, the PlayerIdentifier enum is iteratively
converted to a JavaScript object and passed to the QML-side
through the singleton type "PlayerlistModel" which is
registered with qmlRegisterSingletonType in mainui.cpp.
The new toolbar editor handles the returned object as
 a model and dynamically populates itself per player.

If there is a need to add a new player, just
add a new entry to the PlayerIdentifier enum and
set its identifier accordingly. With the new
approach the new player will automatically
appear in the toolbar editor.

Signed-off-by: Pierre Lamot <pierre@videolabs.io>