textures and meshes in extra projects folder

[Thomas-Feldmeier]

file hierachy

A project should have these folders:

• worlds
• protos
• controllers
• plugins
• textures (new)
• meshes (new)

This would help to seperate .proto and .wbt and .png/.jpg and .obj files.

Textures or meshes are searched in /textures/ or /meshes/ subfolder. Maybe in the future they are not searched in protos or worlds subfolder any more. This would make it less complicated:

https://www.cyberbotics.com/doc/reference/imagetexture#search-rule-of-the-texture-path

see #3922
see #3942

[omichel]

We have a plan to simplify this rule, see #1638.