#12 Entire line erasing

[AHelper]

Kinda large to tackle.

1. Pen with eraser should not render a line
2. Stroke selection detection needs to exist. Probably see how close to the line you are (factoring in the eraser size and each stroke's size.
3. Remove the line from the model
4. Trigger full render
5. Signal that the stroke was completely removed. (probably virtual method in model)