Prevent locked drawings to be deleted. Fixes https://github.com/GNS3/gns3-gui/issues/2948

gns3server/controller/drawing.py

@@ -16,7 +16,6 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 import hashlib
-import asyncio
 import base64
 import uuid
 import re

gns3server/controller/project.py

@@ -684,6 +684,8 @@ class Project:
     @open_required
     async def delete_drawing(self, drawing_id):
         drawing = self.get_drawing(drawing_id)
+        if drawing.locked:
+            raise aiohttp.web.HTTPConflict(text="Drawing ID {} cannot be deleted because it is locked".format(drawing_id))
         del self._drawings[drawing.id]
         self.dump()
         self.emit_notification("drawing.deleted", drawing.__json__())