Rendering status feedback when auto-saving

[njourdane]

The Render button on the buttons-bar has a default state () and a working state ().

This gives a good feedback to the user about the rendering status.

It could be nice to also switch the button state when auto-saving.

[mbway]

This functionality seems very related to the changes I introduced to indicate rendering status in the statusbar. I've included this feature in my pull request #233