Add debounced resize event handler

[swederik]

In OHIF we have a resize handler which is debounced to fire 100ms after resizing stops. This prevents needless re-computation / redraws while the browser is being resized (or the device is being rotated to a new orientation).

The VTK plugin should incorporate something similar. Somehow I think it makes more sense to implement this at the plugin level.

You can call volumeViewport.resize(), as is done here:

VTKPlugin/volumeRendering/main.js

Lines 34 to 39 in 8ca0a96

	// TODO: VTK's canvas currently does not fill the viewport element
	// after it has been resized. We need to set the height to 100% and
	// trigger volumeViewer.resize() whenever things are resized.
	// We might need to find a way to hook onto the OHIF Viewer ResizeManager
	// div.querySelector('canvas').style.height = '100%';
	volumeViewer.resize();

For reference: https://github.com/OHIF/Viewers/blob/master/Packages/ohif-viewerbase/client/lib/classes/ResizeViewportManager.js#L127

[swederik]

Make one PR to OHIF/Viewer:

• ViewportPlugin class should expose overrideable onResize method
• ViewportPlugin class should setup a resize listener in a method _initResizeListener.
• ViewportPlugin should have a default value for debounce time in milliseconds, which any subclass could override
• The handler for this event should check for all plugin divs (similar to https://github.com/OHIF/Viewers/blob/master/Packages/ohif-viewerbase/client/lib/classes/plugins/ViewportPlugin.js#L87) and call onResize with each of them as a the sole argument.

Make another PR to VTKPlugin:

• Adds onResize which calls volumeViewer.resize()