Draft: [REF,FIX] Refactor headerlib to allow ajax append and proper minimization and cache busting

Problems to solve:

1. Tiki doesn't have an organised way to add additional js and css to an already loaded page. This is already happening in a number of cases (previewing wikiplugins, tracker table editor, etc.). It will happen constantly as we transition more and more parts of tiki to a Single Page Application. Currently there is a number of hacks with different shortcomings and bugs.
2. There are multiple cache implementations in tiki. Several of them try to cache parts of headerlib, in different ways (and with different bugs)
3. The js runtime minimization/bundling/cdn handling code is not well isolated. This makes it very hard to make it reliable, optimize, and detect if single single script continuously causes cache misses
4. There are absolutely no unit tests for one of the most core and potentially fragile parts of tiki

• Make the properties of headerlib private
• Add at least unit tests for basic functionnality (ordering, duplicate detection)
• Add snapshot support, and getting a diff since last snapshot. Mostly to support the ajax additional js/css required use cases, and the caches use cases
• Make the hashes of the js added available to javascript
• Isolate the minimization code
• Figure out a way to exclude the hashes already present in the js of the page from the current snaphot
• Make a clean API for all the above
• Finish !4534 (closed)
• Finish !4439
• Finish restoring caches
• Basic review of CDN handling, like !4570 (comment 1796405777)
• Likely load !4147 (merged) from the controller to avoid processing a 3mb js file anytime the feature is enabled

Problems that won't be solved now but will be considerably easier to solve:

1. Complete cache busting in production after a tiki update.

There is much progress being made, but still days of work...