Sync overlays through traits

[bmatthieu3]

This is based on the first cleanup done in #150

This contains:

• fix the documentation examples. overlays do correctly appear on the generated HTML static file because is its passed with traitlets instead of being sent with events.

current online documentation (no overlays are showed):

now:

• if the user wants to export the aladin view to HTML, using embed_minimal_html from ipywidgets does recognize the add of overlays
  To export the aladin object to html through ipywidgets:

from ipywidgets.embed import embed_minimal_html

# drop_defaults is set to False because otherwise content of the widget's _esm is not put in the HTML
# See: https://github.com/manzt/anywidget/issues/339
embed_minimal_html('export.html', views=[aladin], drop_defaults=False)

This gives this html file with the table overlays correctly contained in it:

• add of colormap traitlet to replace the old event
• fix: projection change through the UI now change the value of the python traitlet (aladin.projection)

[ManonMarchand]

It works. My only remarks are:

• the whole overlays are now stored in memory when before they were just sent and forgotten --> maybe this limits a bit the size of overlays (mostly MOCs and tables can get quite big) that we can print.
• it looks like you wanted to allow programmatic deletion of the overlays in this PR. Is this the next step?

[bmatthieu3]

the whole overlays are now stored in memory when before they were just sent and forgotten --> maybe this limits a bit the size of overlays (mostly MOCs and tables can get quite big) that we can print.

I am not sure if this can cause a real issue in practice. I think we should test loading a big MOC, fits image or catalog votable that is currently well handled in master and see if it is not in this branch. If there are problems we could maybe try evaluating the size of the overlay and send an event or not depending on it.
One thing is when exporting to HTML, the HTML will have the overlays encoded inside it which can lead to big html files. We just do that for compiling the docs and the doc examples do not load big overlays.

it looks like you wanted to allow programmatic deletion of the overlays in this PR. Is this the next step?

I think we should do that in another PR maybe ? on the JS side one can call https://cds-astro.github.io/aladin-lite/Aladin.html#removeOverlay with the name of overlay to remove.