Instant eraser completion and code refact

As my compiler was only working with old versions of pencil, I thought it was a local repository problem and I pulled from chchwy to test it and then I pulled from pencil2d but then I synchronised in the ui and the 2 files appeared. I sent it without removing the new commits thinking it's not a problem.

BTW, mingw update solved my compiler problems.

I arranged some code and improved the temprorary eraser, following kaiko's suggest. Now the icon changes immediatly when CTRL+SHIFT are pressed, and eraser minimum size is better adjusted.

Thank you Matt !