Display Artboard Sizes Panel when inserting Art-Board

[AshishS-1123]

Summary / How this PR fixes the problem?

As discussed in #554, this PR implements the feature to display a list of common screen sizes to create a new artboard.

• Created a ArtboardSizesPanel class for displaying size categories in the left panel.
• This component contains screen size categories like Desktop, IPads and Mobiles.
• The component also has a + button that the user can use to create new categories.
• Each category will contain buttons, clicking on which will insert a artboard of the size it mentions.
• The categories have been implemented using Gtk.Expander.
• All the category names and their screen sizes will be saved in the gschema.xml.in file in the form of a JSON string.
• This panel will only be shown when inserting an artboard.
• The newly inserted artboard will be positioned intelligently based on the positions of other artboards

Steps to Test

• Press A to insert artboard.
• In the left panel, you will see a bunch of categories.
• Click on the device category you want to expand it and reveal a bunch of screen sizes
• Clicking on any of these screen sizes will insert an art-board automatically. The canvas also zooms and scrolls to fit the new artboard.
• Click on the + button to create a new category.
• At the bottom of every category, a button is present for adding new screen sizes.

Screenshots

[Alecaddd]

Thank you so much for your contribution. Would you mind waiting until the UI panels are re-enabled in the main branch? We're upgrading to GTK4 and we're not working on the master branch anymore.

[AshishS-1123]

Sure. I close this for now. Though how long do you think it will take for UI and Gtk4 migration to be done?