Improvements to shader editor

[KoBeWi]

This PR does some improvements to shader editor, to bring it closer on par with script editor:

• allow to close shaders with middle click
• tweak shader names to use the same naming as scripts (for built-ins, named shaders etc)
• show (*) for unsaved shaders
• update shader list when scene is saved or shader is modified
  

Putting as draft for now because there is some bug with renaming shaders. Also visual shaders don't have unsaved status, I'll have to see if it's easy to implement.

[KoBeWi]

The bug I mentioned turns out to exist in alpha 13 too, so it's not caused by this PR. I think it should be ready.

I looked into visual shaders, but not sure how to implement the unsaved status. UndoRedo can be used for that, the problem is determining when the shader should be marked as saved. I left a TODO comment, maybe @Chaosus can implement it.

[clayjohn]

Thanks!