Implement multi-select with holding shift key

[StefanOltmann]

At present, on https://photos.google.com/, when you select one picture and then hold the shift key while selecting another, all images between the initially selected one and the clicked one are automatically selected. It would be greatly appreciated if this convenient feature could also be implemented in the desktop version.

[jordond]

I've made some progress on this. I still have a couple things I need to work out. So I haven't pushed the branch yet.