Add option to delete layers from an annotation

[fm3]

#5771 introduced the concept of annotation layers. An annotation can for example have multiple volume annotation layers (which can later be toggled visible vs invisible) – There should be routes and a UI to add a new or delete an existing layer from an annotation.

[fm3]

#6330 added a button to add layers. Renaming this to track the delete layer button

[fm3]

quoting philipp from duplicate #6482:

In case the layer is based on a fallback layer, the naming should probably be "restore original segmentation layer (warning: this will delete all the modifications you applied to it)" or something like that.