Lua plugin: Don't embed external tilesets

Just writes out their name and relative file name. If embedding is
desired, the "Embed tilesets" option can be enabled in the export
preferences.

For those exporting their tilesets as separate files, a relative
"exportfilename" property is also included.

Closes #2120

src/plugins/lua/luaplugin.cpp

@@ -315,12 +315,25 @@ void LuaWriter::writeTileset(const Tileset &tileset,
     }
 
     mWriter.writeKeyAndValue("name", tileset.name());
-    if (embedded)
+    if (embedded) {
         mWriter.writeKeyAndValue("firstgid", firstGid);
 
-    if (!tileset.fileName().isEmpty() && embedded) {
-        const QString rel = mDir.relativeFilePath(tileset.fileName());
-        mWriter.writeKeyAndValue("filename", rel);
+        if (tileset.isExternal()) {
+            const QString rel = mDir.relativeFilePath(tileset.fileName());
+            mWriter.writeKeyAndValue("filename", rel);
+
+            // For those exporting their tilesets separately, it could be
+            // helpful to include a relative reference to the exported file as
+            // well. For consistency I decided to include this separately from
+            // the "filename".
+            if (!tileset.exportFileName.isEmpty()) {
+                const QString rel = mDir.relativeFilePath(tileset.exportFileName);
+                mWriter.writeKeyAndValue("exportfilename", rel);
+            }
+
+            mWriter.writeEndTable(); // tileset
+            return;
+        }
     }
 
     /* Include all tileset information even for external tilesets, since the