Hide outline view if no outline is available

[LeaVerou]

I find the outline view immensely useful for JS code, so I'd rather have it continuously open. However, when I navigate to a CSS file, all I get is a ton of wasted space with the message "No outline available. CSS files do not currently support outlines. Install an IDE package first." (Yes, I know one exists, but that's beyond the point, and also I don't want to install it). This is quite a waste of space, and if I close it, it's closed for all my tabs, even those that do have meaningful outlines. Why display that panel at all if it has no content? Communicating why this is happening to the user could be done with a usual dismissible error message, once per file(type).

[mlavi]

I love outline view as well, but when it is empty, I'd like it to close or collapse after a certain, potentially defeatable time frame so I reclaim unused visual real-estate and remain focused.
Correspondingly, I'd like it to re-open and display when I switch to files that it can help.

[linjialiang]

@mlavi @LeaVerou Is your outline ok?The outline under my computer cannot be loaded correctly

#320