Add getKey() method to ControlObjectSlave.
Since WBaseWidget is common to all skin widgets now, simplify the MixxxApp tooltip event handler.
Use WLabel for the "No OpenGL detected" fallback message.
Add emitOptionToString helper.
Add toDebugString() method to ControlWidgetConnection.
Show debug tooltips when developer mode is enabled.

Provides useful information for skin writers in widget tooltips when in debug
mode.