Refactor server and pixel

1. deplaced main packages in a cmd directory (as it should be done)

2. changed the name of test script to more explicit one

3. refactor server package:

• added a type.go file
• added a file for each type of request (at the moment, only test_request
• small type changes

3. refactor painting package:

• changed the name of the package pixelwar to painting as it is more explicit: this package defines all the stuff to paint on a canvas
• changed the name of the struct playground to canvas as it is more explicit, and reuses the actual r/Place vocabulary
• changed the param of a canvas to heigth, width and grid as it is more explicit

4. change the name of the module to gitlab.utc.fr/pixelwar_ia04/pixelwar to allow go install
5. added a main.go that launches the server to allow go install
6. added some TODOS...