Use external canvas for NavCube

[xeolabs]

Encapsulating management of HTML within JS is an anti-pattern - better to manage HTML externally to the likes of NavCubePlugin etc.

Change NavCube to accept ID of canvas, ie:

new NavCubePlugin(viewer, {
        canvasId: "myNavCubeCanvas",
        visible: true,           // Initially visible (default)
        size: 250,               // NavCube size in pixels (default is 200)
        alignment: "topRight",   // Align NavCube to top-left of Viewer canvas
        topMargin: 170,          // 170 pixels margin from top of Viewer canvas
        cameraFly: true,       // Fly camera to each selected axis/diagonal
        cameraFitFOV: 45,        // How much field-of-view the scene takes once camera has fitted it to view
        cameraFlyDuration: 0.5 // How long (in seconds) camera takes to fly to each new axis/diagonal
    });

[xeolabs]

Now in master