Update Material when MatDef changes on disk

[neph1]

From @grizeldi 's wishlist:

"The preview as it is, is rather useless for shader development, as it doesn’t have live refresh. If I change my shader file on disk, the preview should refresh, so I don’t need to reopen the material window every time I do a small shader tweak. Lack of this functionality sadly prevents me from undocking the material editor, putting it somewhere else in the UI and just start coding."

I see two ways of doing this:

• As the scenecomposer does it for materials, through a listener/provider on the Shader editor
• As the scenecomposer does it for models, listening for changes on the file itself.

The first creates a nice integration with the other editors, encouraging usage. It however requires using them, and I figure a lot of people will be editing text files.
The second allows for external editing, but it's complicated a bit by being multiple files (at least 3, matdef, vertex and frag shaders)

Given that, the first one is probably easier to implement.

[neph1]

If this is going to work in other instances (ie the scenecomposer) using the listener pattern is probably best. Then we can use it wherever we want)

[neph1]

I just found out I had already made a listener while working on the Shader Editor, I presume :P

sdk/jme3-materialeditor/src/com/jme3/gde/materialdefinition/MatStructChangeListener.java

Line 51 in 0435fdd

class MatStructChangeListener implements PropertyChangeListener {

Edit: NM, it was for the structure of the MatDef. But it got me started!