Load menu : add sub-menus to group loaders

When we publish images (for example from Substance Painter), we use Ayon's OIIO to automatically provide several resized versions of the images.
The Ayon Loader allows to choose which of the output images we want to load.

But it displays it as a flat list, which can quickly become too long and difficult to read.

Please provide sub-menu (see my screenshot and mockup gui), to group by load types, to greatly enhance the use of that gui.

Already discussed here :

• Load menu : add sub-menus to group loaders #1132 | AYON Core