Plan to upgrade to Gtk4

[Alecaddd]

With the imminent release of elementaryOS 7, and the consequential stable release of their toolkit upgraded to Gtk4 and all the latest and greatest libraries, it will be finally easier to upgrade Akira to Gtk4.

It will take a while, and since a lot changed in Gtk4, I will need to sort of restart from scratch to learn everything and make sure things are properly implemented along the way.
One of the main hurdle of the first few versions of Akira was bad performance due to bad implementation.

Gtk4 will bring much more simplicity in the code, as I won't need to maintain a custom Virtual Listbox Widget capable of handling thousands of layers without freezing the UI, as well as a better integration with Wayland and all the other shiny new things that have been added in the entire stack.

Since this upgrade won't be happening in a few weeks, here's the plan to ensure things are moving along and you can still test and use upcoming releases of Akira.

Release a couple final alpha versions with Gtk3.

Even if not much activity happened in the past few months, Akira saw a massive rewrite and improvements in the Canvas throughout the year.
A lot of features are still missing as you can see from the list in #358, but only a couple of things are missing in order to release a new Alpha version.

• Fix the Export tool
• Fix the saving and loading of files

These features were temporarily disabled due to the architecture rework of the Canvas.
Once those features are back in, Akira will be usable in its alpha state.

Start rewriting from scratch

While I work on finishing those features, a new gtk4 branch will be created and used to rework Akira from scratch.
This will mean that 2 parallel versions of Akira will be maintained, a Gtk3 version in the main branch, and a Gtk4 version in the gtk4 branch.

This will make it possible to accept PRs and improvements for the currently released alpha, while the ongoing rewriting work happens somewhere else, as well as using the Gtk3 version to compare performance and architecture.

A big Thank you to all the supporters and contributors, as well to all the people interested in Akira.

As usual, it's taking longer than I wished for to bring this application to a usable and stable release, but like everything in the FOSS world, we learn from struggles and we do what we can in our spare time.