GStreamer camera plugin has different convention for videoRecorder.mediaContainer

From CameraRecorder QML type's document:

mediaContainer : string

This property holds the media container to be used for recording video. Typically this is mp4.

However, the value "mp4" or other format extension doesn't work with the GStreamer plugin. Instead, it expects "GStreamer caps" which is platform-specific [1] and isn't documented anywhere.

How to reproduce: starts from declarative-camera example, add the following code in declarative-camera.qml under Camera.videoRecorder. Then, build & start the program.

         videoRecorder {
             resolution: "640x480"
             frameRate: 30
+
+            mediaContainer: "mp4"
         }

Expected result: Video preview works. Video recording works and output files in the "mp4" format.
Actual result: video preview doesn't work. In the terminal the following message is shown:

CameraBin error: "Your GStreamer installation is missing a plug-in."

Actual result with the commit proposed to fix this bug: Video preview works. However, when recording video, the app outputs the file in "ogv" format instead of expected "mp4".

[1] A GStreamer caps can look like a mime, but it isn't. For example, a common mime of mp4 (video/mp4) doesn't work and requires "video/quicktime; variant=(string)iso" instead.