Fixes deserializing rectangle on file load

[mradcliffe]

## Summary / How this PR fixes the problem? - Resolves loading a saved akira file with "rectangle" type. I found akira and started playing around with it, then ran into this bug. I figured out the file format was a zipped json file pretty quickly, opened it up, saw the inconsistency, and the found the issue. I don't really feel comfortable writing in vala, but I felt I would be comfortable enough to create a pull request for this small fix. ## Steps to Test 1. Open Akira 2. Press the Insert button 3. Choose Shapes 4. Choose Rectangle 5. Place the Rectangle 6. Save the file 7. Close Akira 8. Open Akira 9. Open the file Result: the application does not bail with Akira:ERROR:../src/FileFormat/JsonDeserializer.vala:113:akira_file_format_json_deserializer_deserialize_node: assertion failed: (false) ## Known Issues / Things To Do - The rest of #732 is out-of-scope of what I can contribute.

[albfan]

To really avoid the problem, can you do another commit to change all those Magic words into constants?

[mradcliffe]

can you do another commit to change all those Magic words into constants?

I reviewed to see if I could handle this. Is like the best place for this

• enum constants within Akira.Lib.Items.Model?
• constant in each Akira.Lib.Items.ModelTypeThing?
• enum constants in one of the Managers?

Regardless it seems like model name_id method should return that constant as well.