Explain * and ∅ in Layer info window

[Klumbumbus]

The new characters from #12326 should have an explanation. (While * is well-known, ∅ not so much I think.)
I made a patch, please review. It creates no additional i18n string and changes the term "document" to "layer". "document" was a bit strange.

OsmDataLayer.java

@@ -710 +710 @@
         addConditionalInformation(p, tr("Download is blocked"), data.getDownloadPolicy() == DownloadPolicy.BLOCKED);
         addConditionalInformation(p, tr("Upload is discouraged"), isUploadDiscouraged());
         addConditionalInformation(p, tr("Upload is blocked"), data.getUploadPolicy() == UploadPolicy.BLOCKED);
+        addConditionalInformation(p, "\u2205 " + tr("Empty layer"), this.getDataSet().isEmpty());
+        addConditionalInformation(p, "* " + tr("Unsaved changes"), this.isDirty());
 
         return p;
     }
@@ -1214 +1216 @@
         if (isDataSetEmpty() && 1 != GuiHelper.runInEDTAndWaitAndReturn(() ->
             new ExtendedDialog(
                     MainApplication.getMainFrame(),
-                    tr("Empty document"),
+                    tr("Empty layer"),
                     tr("Save anyway"), tr("Cancel"))
-                .setContent(tr("The document contains no data."))
+                .setContent(tr("The layer contains no data."))
                 .setButtonIcons("save", "cancel")
                 .showDialog().getValue()
         )) {

[simon04]

Great, thank you!

[simon04]

In 17757/josm:

fix #20750 - Explain * and ∅ in Layer info window (patch by Klumbumbus, modified)

[simon04]

In 17763/josm:

see #20750 - Fix OsmDataLayerTest

[Klumbumbus]

Thx.