Work on treeview.
* Added some mixin items to allow us to implement the PsiNamedElement interface on relevant items.
* Built some infrastructure around the actual tree implementation
Work on treeview.
* Moved code for finding descendants onto psi elements.
Work on treeview.
* Changed interface from being mandatory for items to being a marker interface with a single method.
Work on treeview.
* Removed Atlassian from headers
Merge branch 'master' of https://github.com/Vektah/idea-rust into treeview

Conflicts:
	src/java/main/vektah/rust/ide/sdk/RustSdkData.java
Couple of small fixes:
1. Don't show expansions for structure view
2. Show method icon for methods
Fixed up broken parser tests.
Merge pull request #13 from Myrannas/treeview

Structure view implementation