Add layer loading spinner and remove layer button

[giswqs]

This PR adds a layer loading spinner and remove layer button to the layer manager. The layer widget padding is slightly reduced from 8px to 4x to make room for the two newly added buttons.

layer_loading_spinner.mp4

[github-actions]

🚀 Deployed on https://6669cc1c9992cc56d524e133--opengeos.netlify.app

[giswqs]

Just realized that I forgot to add a confirmation dialog when removing a layer. Will add that later

[giswqs]

I have added the confirmation dialog when removing a layer. The PR is ready for review now.

spinner.mp4

[naschmitz]

Looks great! Thanks for taking this on!

[naschmitz]

We shouldn't import ipyleaflet in map_widgets. This file is supposed to be agnostic to map implementation. Two possible solutions are listed below.

1. The preferred option is to avoid this problem entirely by showing the confirmation dialog above (or inside) the existing widget. It could look like another row that's added below the row scheduled for deletion or a dialog that appears over everything.

2. Build this new "confirmation" widget as any of the other core widgets and have the widget control code live in core. The new widget needs some sort of "on_confirm_deletion" event that the code code must set. Example:

   geemap/geemap/core.py

   Line 683 in b115dfb

   def _add_basemap_selector(self, position: str, **kwargs) -> None:

[giswqs]

I have implemented the preferred option 1:

another row that's added below the row scheduled for deletion

remove.layer.mp4

[giswqs]

The remove layer confirmatition widget is now being displayed beneath the layer row in the layer manager. See the new demo:

remove.layer.mp4

[giswqs]

@naschmitz @jdbcode It is ready for review now.