Comment 8 for bug 661460

I am not sure if I get the issue correct, but I think it would be the best to separate the crates and smart crates.

Think of crates in terms of file system folders containing symlinks to the original files, manually setup by the user.
We may allow to manually group crates or create a new crate within a upper crate.
It sounds reasonable to limit this to a tree structure only, just like it would be in a file system.
.. or like a vinyl DJ that puts a sorter into his real crate.

Smart crates are for me sticky filtered filtered views of the library. They do not need to be applied on other filtered views.
They also do not need to apply on Crates or playlists, if we allow to set a filter "member of crate/playlist" Bug #1402133
Maybe we should name it not smart crate.

A related aspect of this bug is the hierarchical view of the library. We have some bugs that propose different nature of such a hierarchical tree.
I have collected the browse related bugs here: Bug #412453
Here is the multi level crate Bug #671632 and virtual folders: Bug #1228789

For me, a complete solution will feature this:
* Smart crates = Sticky filters (focused by this bug)
* A hierarchically organized Library view
** by file location (in addition to our RAW browse feature)
** by Artist -> Album -> Track
* Crates tree, allow to put on or more crate into a Meta crate
** allows to build genre trees