WIP: Saving and loading akira through serialization

[albfan]

Summary / How this PR fixes the problem?

Use existing serialization:

• https://valadoc.org/json-glib-1.0/Json.gobject_serialize.html
• https://valadoc.org/json-glib-1.0/Json.gobject_deserialize.html

to save and load canvas contents.

Steps to Test

This is just a PoC so it only saves first item from canvas and loads it as a rectangle.

Example file (you can generate other saving your canvas):

file.akira

{"line-width":1.0,"stroke-color-rgba":2863311615,"fill-color-rgba":3435973887,"x":699.6171875,"y":67.75390625,"width":104.0,"height":97.0}

• It needs to traverse all items on save
• Seems serialization has problems with CanvasItem (cannot recognize as a type), so it will need to store type (Rect, Ellipse, Text) for each item to deserialize

Screenshots

Known Issues / Things To Do

• Add akira filter to open/save dialogs
• Test load items order

This PR fixes/implements the following bugs/features:

Relates to #90. A svg format would be preferred anyway.

[albfan]

First functional format:

{
  "items": [
    {
      "type": "GooCanvasRect",
      "item": {
        "line-width": 1,
        "stroke-color-rgba": 255,
        "fill-color-rgba": 1318717183,
        "x": 104.32421875,
        "y": 70.765625,
        "width": 282,
        "height": 232
      }
    },
    {
      "type": "GooCanvasEllipse",
      "item": {
        "line-width": 1,
        "stroke-color-rgba": 255,
        "fill-color-rgba": 4012452351,
        "center-x": 368.61328125,
        "center-y": 305.26171875,
        "radius-x": 147.5,
        "radius-y": 145.5,
        "x": 221.11328125,
        "y": 159.76171875,
        "width": 295,
        "height": 291
      }
    }
  ]
}

• Mark objects to not save (as hover and select effect)

[albfan]

@AUNaseef please keep in mind this is a tentative design (mostly a PoC that we can discard early), but can you check if you can save/load your draws with this branch?

I would love to load any of your draws and take a look. (Specially for the group feature)

[albfan]

• store transformation: Now it is missing. It should be just store and load the transform matrix
• Group objects. Just store the parent item? It will be done automatically? What if parent do not exists while loading?
• Mark special objects (like artboards) Includes grouping and intersections (invisible parts for items inside and artboard

[albfan]

• Add artboard layers on load

[albfan]

• save json in pretty format