.material suffix for shaders should be changed to .shader

Having .material to indicate shader and .materialdef to indicate actual usable material and storing both in the same directory as well is way too confusing for the user. It is difficult to discern at a glance which files are materials and which are shaders, especially if the file is not in use and is therefore hard to read gray. 

We should introduce alternate shader file suffix .shader and rename our library materials to use that.  We still should support .material suffix for backward compatibility, though.