Add painting layer

[hanslovsky]

We still do not have a painting layer which was one of the core components of the old BigCAT. Eventually, we want a multi-resolution painter that paints at the appropriate resolution. For a quicl start, add a simple painter for the highest resolution only.

[hanslovsky]

Small update:
Just about 30 minutes ago, I was able to paint (at a single location) into each of the ortho view and all ortho views were updated. There still seems to be a scaling issue with the an-isotropic views but that is work for tomorrow. I am hopeful that I will be able to paint actual lines tomorrow. Once this is ready, I will focus on multi-scale painting and "commiting" or merging down into the actual label data.

[hanslovsky]

For now, I decided to draw 2d circles (instead of 3d spheres), as we have orthogonal views and can paint into any dimension anyway. This requires that either (a) all ortho views are locked in their "native" plane (xy, yz, zx), or (b) the circles are rotate in space, accordingly. I prefer (a) but I have not implemented that, yet (should not be too much work). At least for anisotropic data, I think that this is a good solution, at least until imglib/imglib2-algorithm#23 is resolved.
Of course, this is still up for discussion and no final decision has been made. Offering both options (2D and 3D) to the user seems reasonable to me.

[hanslovsky]

Progress report:

• Done
  • Basic painting exists with
    • drag/click paint
    • flood fill
    • intersection of canvas with background
• TODO
  • Multi-res generation when saving canvas
  • (Currently) incompatible with the 3D rendering changes proposed in Use only one instance of SelectedIds per source #63. However, this should be fairly easy to fix (i.e. re-populate the blocks of the mesh cache that are affected by painting).

[hanslovsky]

Development for bigcat.v2/paintera continues in a dedicated repository now. Check saalfeldlab/paintera#1