Adding ability to add or remove points from existing polygon when left-clicking while holding down the SHIFT-key.

I found this is not very useful especially when we're annotating
concave-shaped objects.

Windows by default uses GB2312.

For old files, you can use iconv:

  iconv -f gb18030 -t utf-8 1.json > 1_utf8.json

https://github.com/wkentaro/labelme/issues/687