Cargo: dependency management

[vosen]

Split off from #3.
Currently, we don't handle Cargo dependencies at all. We will support Cargo dependencies by having nice "Package management" window a la "Manage Nuget Packages" (we should copy it wholesale as nuget VS extension is apl2-licensed). Also added dependencies will show up in "References" node with some quick info.

[mattiasflodin]

areweideyet.com gives the appearance that Visual Studio support is very advanced. But honestly, without ability to install third-party dependencies you can't do anything practically useful at all, can you? Or is there a suggested workaround for this?

[MaulingMonkey]

Current versions let you modify Cargo.toml files yourself and build against those dependencies OK. We just don't have any UI for it.