[Usablity]Should define consistant message box for both engines:helix and gst

[dandan.hong@intel.com]

Test on version 0.21 with gstreamer-dbus-media-service 0.1.10ubuntu1 and helix-dbus-server_0.21-alpha2,error message box prompted by helix engine and gstreamer engine are of different style.So suggest that define consistant message box for both helix and gstreamer engine.

Reproduce steps
1.Distable helix engine and enable gst engine
2.Playback a codec which are not supported by with gstreamer,for example MID_AAC.mp4
3.Distable gstreamer engine and enable helix engine.
4.Playback a codec which are not supported by helix,for example MID_AC3.mp3

Current result:
At step 2,there prompts an mesage box says that "Missing plugin MPEG4-AAC decoder.Please install this plugin".
At step 4,it promps an message box says that "HXR_INVALID_FILE".

Expected result:
All message boxes or dialogs consist with the same style.

[Rusty Lynch (rusty-lynch-intel)]

Since we now only support using one media service at a time, this bug is not visible to the end user.

But... it's still a problem

[Churchy Zheng (changqiu-zheng)]

verify it as only one engine is enable at the same time. this issue is gone.