Add a Plus (Target) icon

[acasteigts]

It would be nice to add the following icon to the Icons class after renaming it as "target.png":

https://www.labri.fr/perso/acasteig/teaching/algomob/practice/files/_target.png

[remikey]

This image is fairly simple and could be seen as a "plus" or simply a "cross". "target" seems tied to one specific usage.
I suggest naming the png cross.png or plus.png and providing the following constants:

• Icons.CROSS
• Icons.PLUS
• Icons.TARGET

What do you thing @acasteigts ?

[acasteigts]

OK for plus.png and Icons.PLUS. For now, let us use a single constant per icon. Thanks.

[remikey]

merged into develop.