Whiteboard icons with night mode

Yet another version of the whiteboard icons.
The main point is that these bring a long a white version that works in night mode.

The style is quite different from the current ones (see  #474, #472). Those are hand-drawn, which is a nice idea, but i like the recommended style "[pictographic, flat, not too detailed](https://developer.android.com/design/style/iconography.html)".