Re-structure the repo into a cargo workspace instead.

Only created the workspace, didn't split the code.

The crates would treat each other as completly different and you have use extern crate fractal-api to import it like if it were a cargo crate.

In order to run now the binary/gtk-client instead of cargo run now you do cargo run -p fractal-gtk. The -p flag specifies wich crate you want to target.