Add an "import image" module

[jywarren]

A module that fetches a remote image via URL (or data URL) and overlays it on the canvas (at a given x, y, w, h ?)

Based on the initial load image code:

https://github.com/publiclab/image-sequencer/blob/master/src/ui/LoadImage.js

Plus the UI code from the demo for selecting an image: https://github.com/publiclab/image-sequencer/blob/master/examples/lib/imageSelection.js

This should allow selecting a 2nd image, and overlaying it over the first.

Later, we could offer "blend modes" between the previous image and the new one we're overlaying, pending access to earlier steps' image outputs.

[jywarren]

started with generic image selector in #253

[jywarren]

Solved in #263 !!!