Tree view improvements

[artonge]

After a discussion with @susnux, here is a list of improvements that we could do for the tree view in the sidebar:

1. Only load depth 1 for the initial request, then depth 2
2. No need to load depth 1 if we are on the root
3. Register in Views batch
4. Register only the visible Views, and register the to be visible ones when we open a folder
5. Don't open the tree view by default
6. The loading indicator is behaving weirdly
7. Remove the hidden Views from the DOM
8. Share folder content with breadcrumbs
9. When we load the tree view from a subfolder, it is not possible to display the siblings up to the root folder

• Easy and impact full: 1, 3, 4
• Harder and impact full: 8
• Nice to have: 2, 5, 6, 7

@susnux is currently looking at 1, 3 & 4 for v33, the rest is only for posterity and if we have the time to do it.