Show notification when using shortcut to change current selected labe…

…l to largest used label plus one (#6546)

# References and relevant issues

Closes #6517 

# Description

Add showing a notification when the shortcut to set current selected
label to largest used label plus one can't change the label value since
it is already the largest used label plus one.

A preview:


![label](https://github.com/napari/napari/assets/16781833/e62a11e3-21dc-4cba-bc54-6e671e2d8aa8)

---------

Co-authored-by: Peter Sobolewski <76622105+psobolewskiPhD@users.noreply.github.com>
Co-authored-by: Grzegorz Bokota <bokota+github@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

napari/layers/labels/_labels_key_bindings.py

@@ -76,10 +76,21 @@ def activate_labels_erase_mode(layer: Labels):
 def new_label(layer: Labels):
     """Set the currently selected label to the largest used label plus one."""
     if isinstance(layer.data, np.ndarray):
-        layer.selected_label = np.max(layer.data) + 1
+        new_selected_label = np.max(layer.data) + 1
+        if layer.selected_label == new_selected_label:
+            show_info(
+                trans._(
+                    "Current selected label is not being used. You will need to use it first "
+                    "to be able to set the current select label to the next one available",
+                )
+            )
+        else:
+            layer.selected_label = new_selected_label
     else:
         show_info(
-            "Calculating empty label on non-numpy array is not supported"
+            trans._(
+                "Calculating empty label on non-numpy array is not supported"
+            )
         )