Fix equation numbers after row operation (#12665)

After removing/adding rows, the corresponding equation numbers in
the LyX display were not updated (although if you clicked outside
the math inset they were).

Now they are updated immediately after the operation.

src/mathed/InsetMathGrid.cpp

@@ -1550,6 +1550,7 @@ void InsetMathGrid::doDispatch(Cursor & cur, FuncRequest & cmd)
 			cur.undispatched();
 			break;
 		}
+		cur.forceBufferUpdate();
 		// perhaps this should be FINISHED_BACKWARD -- just for clarity?
 		//lyxerr << "returning FINISHED_LEFT" << endl;
 		break;