Loader: Add submenu to group loaders in load context menu:

[BigRoy]

Changelog Description

Quick and dirty prototype to test whether grouping of loader actions is better UX or not

An env var AYON_LOADER_GROUP_MODE can be used to switch between never, multiples, always. The default is now "always". If an invalid value is set for the env var not from the aforementioned list it will silently use always

Additional info

This is merely a draft prototype so one can test whether it actually improves the UX.

There are some issues:

• Submenu actions don't seem to get highlighting on mouse over
• The context menu on the individual representations bottom right in the loader UI are also always grouped, even though there should never appear 'duplicates' in that list.
• The "label" for the menu items is currently very hackily 'patched' from the action item by splitting of the last " (" which definitely isn't what we should do.

This may completely be superseded by way more thorough API overhaul: #1058

This is not production ready ☝️ ; it's merely intended for cosmetically testing the change

Intends to fix #1132

Testing notes:

1. Play around and feedback on it cosmetically.

[alainxi]

Works for me, until I try to click on the option icons (when get_options is implemented).
The icon appears in the menu, but it has no effect (the loader behaves as if I clicked on the menu, not on the icon), the option popup does not appear.