Material editor doesn't support animated materials well

[grizeldi]

Currently material editor doesn't support animated materials, as it only redraws when a parameter is updated. This makes editing of shader animated materials rather cumbersome.

[neph1]

We should be able to reuse the code for the shader editor to do this.

[neph1]

MaterialPreviewRenderer is the class responsible for showing materials.
BackdropPanel, which is used for shader nodes, already implements a timer for continuous rendering.
com.jme3.gde.materialdefinition.editor.MatPanel has a similar role for Material editor, but no continuous rendering.

Suggestion is to:

• Implement a similar timer in MatPanel, responsible for updating the preview.

• Add a button to MaterialEditorTopComponent for toggling animation on and off.