Implement Syncback to support converting Roblox files to a Rojo project (#937)

This is a very large commit.
Consider checking the linked PR for more information.

rojo-test/syncback-tests/ref_properties_duplicate/input-project/default.project.json

@@ -0,0 +1,6 @@
+{
+  "name": "ref_properties_duplicate",
+  "tree": {
+    "$path": "src"
+  }
+}

rojo-test/syncback-tests/ref_properties_duplicate/input-project/src/Pointer_1.model.json

@@ -0,0 +1,6 @@
+{
+  "className": "ObjectValue",
+  "attributes": {
+    "Rojo_Target_Value": "an ID that should not change"
+  }
+}

rojo-test/syncback-tests/ref_properties_duplicate/input-project/src/Pointer_2.model.json

@@ -0,0 +1,6 @@
+{
+  "className": "ObjectValue",
+  "attributes": {
+    "Rojo_Target_Value": "an ID that should not change"
+  }
+}

rojo-test/syncback-tests/ref_properties_duplicate/input-project/src/Target.model.json

@@ -0,0 +1,6 @@
+{
+  "className": "Folder",
+  "attributes": {
+    "Rojo_Id": "an ID that should not change"
+  }
+}