core ui: the IVideoControl is used outside of the MediaPlayerElement (e.g. VideoView)

Fixes #364

src/LibVLCSharp.Uno/VideoView.UWP.cs

 using System;
 using LibVLCSharp.Platforms.UWP;
-using LibVLCSharp.Shared.MediaPlayerElement;
+using LibVLCSharp.Shared;
 
 namespace LibVLCSharp.Uno
 {

src/LibVLCSharp/Shared/MediaPlayerElement/IVideoControl.cs → src/LibVLCSharp/Shared/IVideoControl.cs

 using System;
 
-namespace LibVLCSharp.Shared.MediaPlayerElement
+namespace LibVLCSharp.Shared
 {
     /// <summary>
     /// Interface for video control