Reimplement Save and Load file

[Alecaddd]

Now that we switched to the main branch, in order to create a new Flatpak and elementaryOS release, we need to make sure the saving and loading of files works as expected.
Right now it just crashes, but it shouldn't be too difficult to make it work again as all the methods are in place.

[mbfraga]

Reasons for assertions atm:

1. "rect" should be changed to "rectangle" in JsonDeserializer.vala
2. serialization/deserialization for path was never implemented.