Add an highlighter tool/layer

[tokazio]

Resolving https://github.com/mbrlabs/Lorien/issues/242

Add a simple highlighter tool drawing on a layer at level 0 Other tools drawing on layer at level 2 Older versions are on layer at level 0

File version is now 3:

• 1 is the older one
• 2 is handling stroke's layer
• 3 is handling alpha on stroke color

Color and size selection stay the same but is applied:

• size is the selected one *10 for the highlighter
• color is the selected one with alpha 0.8 for the highlighter

The highlighter should have its own size/color in another iteration