highlighter tool

[tokazio]

Resolving #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

[mbrlabs]

Thank you for your contribution!

At this point though i don't want anything that involves multiple layers in the program. I'd first have to implement a proper extensible layer system to avoid hardcoded layers like this.
I am also not convinced this feature is worth the added complexity, so closing this.