Make the name field in projects optional (#870)

Closes #858.

If a project is named `default.project.json`, it acts as an `init` file
and gains the name of the folder it's inside of. If it is named
something other than `default.project.json`, it gains the name of the
file with `.project.json` trimmed off. So e.g. `foo.project.json`
becomes `foo`.

src/snapshot_middleware/snapshots/librojo__snapshot_middleware__project__test__no_name_project.snap

@@ -0,0 +1,19 @@
+---
+source: src/snapshot_middleware/project.rs
+assertion_line: 725
+expression: instance_snapshot
+---
+snapshot_id: "00000000000000000000000000000000"
+metadata:
+  ignore_unknown_instances: true
+  instigating_source:
+    Path: /foo/default.project.json
+  relevant_paths:
+    - /foo/default.project.json
+  context:
+    emit_legacy_scripts: true
+name: no_name_project
+class_name: Model
+properties: {}
+children: []
+