Add an ability to unassign textures in material editor

[mrDIMAS]

Currently there's no way to unassign a texture property in the material editor and the placeholder grid texture cannot be removed from it. Sometimes it is useful to not have any texture assigned and just use the default fallback sampler value (white 1px texture). There should be a new option in the context menu for texture properties - Clear (or something similar). The context menu is located in editor\src\material.rs.