Why this PR ?

• Having a way to save as a new version without overriding the current file
• Having a more modern save UI standardized through different platforms

I was mainly inspired by Blender save UI :

Description of the content

Create a new file manager :

• Left part with standard directories and favourites
• Add right part with
  • search toolbar
  • file list view
  • open/save toolbar
• the bottom toolbar is the main new addition :
  • there's a "+" sign that changes the name of the file to add an index so that we can save a new version
  • if the file exists the rectangle is colored in red to have a visual feedback

Next steps

If reviewers are okay with this proposal there's a few changes to add still :

• Replace the Open/Save as with this UI by default
• Add buttons to go to previous location, create folder
• Add button to filter files and filter to ".mg" by default
• ...