Add the class workspaces
Ajout de la branche bf-workspace
Update the item_instance loaders to use the new image pool.
Updates bf::item_field_edit to use the new image pool.

The new class is_visual_type is used to metaprogramatically select the functions
related to the visual data types (sprites, animations).
Correction de animation-editor [terminé].
Correction de model-editor.
L'éditeur d'animation fonctionne
Correction du model-éditor
The name of the default workspace can be given to the editor with the --workspace option.
Ajout d'un item_class_pool dans workspace_environment
Remove the folder of spritedesc-conv. We use pack-my-sprites.
Les membre de workspace_environment sont privés
Merge branch 'bf-workspace' of github.com:j-jorge/bear into bf-workspace
Compilation de l'éditeur de niveau
Fix mouse event processing in the engine.

The button up events where not processed. Each button down event
was thus followed by a button up, even if the button was not
released.

Add a background in the field box of the item_information_layer.
Fix the direction of the items' system in the wireframe layer.
bear::wireframe_layer displays the internal force.
Remove useless namespace naming.
Add a method in bear::universe::world to get the sum of the forces applied to an item.
Experimenting more precise collision for the slopes.
The items are aligned on the slopes exactly where they crossed the ground.