Create a module to provide JBotSim's "default" images

[remikey]

Try and do the following:

• extract relevant images from existing modules and examples
• create a module which allows developpers to access those images
• use resources from the new module in the modules and examples

[remikey]

Clarification: this module should only contain default provided icons.

[remikey]

As discussed in #36 , the current default icon for nodes (circle.png) should be duplicated and called default-node-icon.png.

[remikey]

Applied on branch feature/26-icons-as-module.
What's been done:

• create jbotsim-icons submodule;
• create default-node-icon.png from circle.png, used by JNode;
• move relevant (and resized when over 10ko) icons from examples to jbotsim-icons.

Current icons list:

• canadair.png
• car.png
• cherry.png
• circle.png
• default-node-icon.png
• drone.png
• farmer.png
• fire.png
• sensor.png
• sheep.png
• station.png
• wolf.png

[remikey]

Merged into develop.

[remikey]

Merged into master.