Loading indicator for layers

[SylvainCorlay]

Some layers may be slow to render, such as geotiff layers when the data is large. This is the case with geotiff.jgis in the JupyterLite deployment.

I figured it may be useful to have some visual indication that the map currently displayed is still incomplete.

• Maybe a spinning wheel or other loading indicator on layers that are not yet available in the layers list?
• Maybe a similar indicator in a corner of a map?

[mfisher87]

I noticed for layers that are slow to load that during loading the layer visibility toggle buttons can change state visually (e.g. the icon changes), but the map is unaffected. At first I thought I had gotten JGIS into an unrecoverable state, but then the layer appeared and the buttons began working again. A loading indicator would really help! 💯

[martinRenou]

Closed by #348