Adding Marker and TimedText Virtual Tracks

[IMFTool]

• Adding TimedTextVirtualTrackType, @dtatut: addresses #17
• Adding MarkerVirtualTrackType, @Ulrich-DPP: addresses #21
• Example instance with TimedText and Marker Virtual Track added
• Netflix example modified to match the updated schema
• EssenceEncoding values constrained for Image and Audio Virtual Tracks

Please note that the position of the EssenceEncoding element has changed for AudioVirtualTrackType and ImageVirtualTrackType. Existing XML instances need to be modified accordingly.