Do not allow edit mode for read-only layers in the feature form

[tomasMizera]

Our current way of handling read-only layers is not nice.

If a layer is identifiable, but read-only we allow users to preview the feature form. This is ok, but we also allow them to hit edit and change the values of fields.

When a user clicks on save in the form, nothing happens - not even a message. Console prints:

 QGIS Input: Warning: Cannot start editing

When a user tries to delete the feature, we show a generic error window saying "this should not happen normally". In this case the console says:

"2023-10-23T09:36:32.563Z QGIS Input: Warning: Cannot start editing\n"
"2023-10-23T09:36:32.563Z CommitChanges: Failed to commit changes:\nERROR: layer not editable\n"
"2023-10-23T09:36:32.564Z QGIS Input: Warning: Cannot delete feature\n"

Expected behaviour
We need to hide the edit button for features in read-only layers

To consider: how about relations? If the parent layer is read-only, shall we allow hitting the "add" button to add a child feature if the linked layer is editable?

[wonder-sk]

To consider: how about relations? If the parent layer is read-only, shall we allow hitting the "add" button to add a child feature if the linked layer is editable?

I would say yes, if the linked layer is editable, we should show allow add button to add features in the linked layer, to keep things conceptually correct, even though it may seem a bit strange at first :-)