[Example] Add example of Canva Editor

[davidalmarinho]

Description

Add a new example, a canvas editor.
In a project that I am working on it, I spent some hours replicating the canvas editor of imgui example demo window in java because have to read cpp code since have to use List intestead of ImVector<>. So, I think that is a example that is worth it having in this repo.

Fixes # (issue)
No fixes.

Type of change

Please check options that are relevant.

• Minor changes or tweaks (quality of life stuff)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
  Didn't add tests, but tested the code and it is working properly.
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[SpaiR]

@davidalmarinho Thank you for the contribution!

I've made some notices about the code. Please, address them to proceed. Also, would you kindly to rebase your branch to the latest main? That is required to launch CI checks.

[davidalmarinho]

You are welcome!
I think that now is all corrected.
If you need something else let me know :)