Implement support for Overlays

[Nour-Cheour10]

No description provided.

[martinRenou]

Thanks! I have a suggestion for improving the implementation, but let's merge and iterate!

[martinRenou]

This approach may be fine for now, but it does not scale well if you have many types of overlays. The BaseOverlayView implementation front-end side becomes too complicated with a big if condition and many different methods depending on the type.

It would be nicer to have one widget implementation per overlay type:

Suggested change

	class BaseOverlay(DOMWidget):
	_model_name = Unicode('BaseOverlayModel').tag(sync=True)
	_model_module = Unicode(module_name).tag(sync=True)
	_model_module_version = Unicode(module_version).tag(sync=True)
	_view_name = Unicode('BaseOverlayView').tag(sync=True)
	_view_module = Unicode(module_name).tag(sync=True)
	_view_module_version = Unicode(module_version).tag(sync=True)
	overlay_type = Unicode().tag(sync=True)
	position = List([0, 0]).tag(sync=True)
	class ImageOverlay (BaseOverlay):
	overlay_type = Unicode('image').tag(sync=True)
	image_url = Unicode('').tag(sync=True)
	class VideoOverlay (BaseOverlay):
	overlay_type = Unicode('video').tag(sync=True)
	video_url = Unicode('').tag(sync=True)
	class PopupOverlay (BaseOverlay):
	overlay_type = Unicode('popup').tag(sync=True)
	popup_content = Unicode('').tag(sync=True)
	class BaseOverlay(DOMWidget):
	_model_module = Unicode(module_name).tag(sync=True)
	_model_module_version = Unicode(module_version).tag(sync=True)
	_view_module = Unicode(module_name).tag(sync=True)
	_view_module_version = Unicode(module_version).tag(sync=True)
	overlay_type = Unicode().tag(sync=True)
	position = List([0, 0]).tag(sync=True)
	class ImageOverlay (BaseOverlay):
	_view_name = Unicode('ImageOverlayView').tag(sync=True)
	_model_name = Unicode('ImageOverlayModel').tag(sync=True)
	overlay_type = Unicode('image').tag(sync=True)
	image_url = Unicode('').tag(sync=True)
	class VideoOverlay (BaseOverlay):
	_view_name = Unicode('VideoOverlayView').tag(sync=True)
	_model_name = Unicode('VideoOverlayModel').tag(sync=True)
	overlay_type = Unicode('video').tag(sync=True)
	video_url = Unicode('').tag(sync=True)
	class PopupOverlay (BaseOverlay):
	_view_name = Unicode('PopupOverlayView').tag(sync=True)
	_model_name = Unicode('PopupOverlayModel').tag(sync=True)
	overlay_type = Unicode('popup').tag(sync=True)
	popup_content = Unicode('').tag(sync=True)

This means implementing an ImageOverlayView, ViewOverlayView and PopupOverlayView front-end side.

Let's keep this comment for a future PR and merge.