Support features without geometries in editor

[steff-o]

Is your feature request related to a problem? Please describe.
The edithandler assumes that a feature and the layer has a geometry and a geometry type for some operations. Currently it is almost impossible to get into a situation where that is a problem, but if #1615 is implemented or someone is adding features fr

Describe the solution you'd like
Make all code in the edit flow honor the isTable setting on a layer and not require a geometry. (Or by other means allow features without geometry where it makes sense)

Describe alternatives you've considered
Adding a dummy geometry to layers without geometry. But it seems rather silly when we already have the isTable setting.

Additional context
This is a concretization of the ideas in #1613