Ensure the self.filenames list is sorted internally.

This is important because even though the QListWidget UI is sorted
automatically, the list is not necessarily. Drag-drop events, depending
on the order in which items were highlighted before being dragged,
can result in a different or reversed order. This has implications for
popping the list later (e.g on delete events)

onionshare_gui/file_selection.py

@@ -136,6 +136,8 @@ class FileList(QtWidgets.QListWidget):
                 return
 
             self.filenames.append(filename)
+            # Re-sort the list internally
+            self.filenames.sort()
 
             fileinfo = QtCore.QFileInfo(filename)
             basename = os.path.basename(filename.rstrip('/'))