Dap: move gtk calls into the gui thread when creating DapInfoBar
Dap: move gtk calls into the gui thread when creating DapInfoBar

Place the creation of DapInfoBar a bit later in DapSource::Initialize(),
where there is another ThreadAssist.ProxyToMain() call for the creation
of the DapContent object, so we reuse the proxy operation for the call
to both constructors.

Before this change, lots of gtk calls were reported by gui-thread-check, such as:

*** GTK CALL NOT IN GUI THREAD: Widget.gtk_widget_queue_draw
   Widget.QueueDraw
   SegmentedBar.UpdateSegment
   DapInfoBar.UpdateUsage
   DapInfoBar.BuildWidget
   DapInfoBar..ctor
   DapSource.Initialize
   MassStorageSource.DeviceInitialize
   DapService.FindDeviceSource
   MapDeviceJob.Run
   Scheduler.ProcessJobThread
   Thread.StartInternal

*** GTK CALL NOT IN GUI THREAD: Box.gtk_box_pack_start
   Box.PackStart
   DapInfoBar.BuildWidget
   DapInfoBar..ctor
   DapSource.Initialize
   MassStorageSource.DeviceInitialize
   DapService.FindDeviceSource
   MapDeviceJob.Run
   Scheduler.ProcessJobThread
   Thread.StartInternal

stsundermann/banshee,ixfalia/banshee,stsundermann/banshee,Dynalon/banshee-osx,Dynalon/banshee-osx,GNOME/banshee,babycaseny/banshee,stsundermann/banshee,ixfalia/banshee,Dynalon/banshee-osx,ixfalia/banshee,Dynalon/banshee-osx,arfbtwn/banshee,Carbenium/banshee,dufoli/banshee,GNOME/banshee,ixfalia/banshee,GNOME/banshee,dufoli/banshee,Dynalon/banshee-osx,stsundermann/banshee,ixfalia/banshee,ixfalia/banshee,ixfalia/banshee,dufoli/banshee,dufoli/banshee,arfbtwn/banshee,arfbtwn/banshee,babycaseny/banshee,dufoli/banshee,GNOME/banshee,Carbenium/banshee,arfbtwn/banshee,stsundermann/banshee,arfbtwn/banshee,GNOME/banshee,Dynalon/banshee-osx,GNOME/banshee,Dynalon/banshee-osx,Carbenium/banshee,babycaseny/banshee,stsundermann/banshee,arfbtwn/banshee,Carbenium/banshee,Carbenium/banshee,arfbtwn/banshee,Dynalon/banshee-osx,babycaseny/banshee,stsundermann/banshee,babycaseny/banshee,GNOME/banshee,arfbtwn/banshee,Carbenium/banshee,dufoli/banshee,stsundermann/banshee,dufoli/banshee,dufoli/banshee,babycaseny/banshee,babycaseny/banshee,GNOME/banshee,babycaseny/banshee,ixfalia/banshee