Re: [pgadmin4][patch] Initial patch to decouple from ACI Tree

As of right now, our Tree abstraction acts as an adapter to the aciTree library. The aciTree library needs the domElement for most of its functions (setInode, unload, etc). Thus this is the easiest way to introduce our abstraction and keep the functionality as before - at least until we decide that whether we want to switch out the library or not.

Since this tree is referenced throughout the codebase, considering it to be a singleton seems like the most appropriate pattern for this usecase. It is very much the same way how we create a single instance of the aciTree library and use that throughout the codebase. Moreover, it opens up opportunities to improve performance, for example caching lockups of nodes. I’m not a fan of singletons myself, but I feel like we’re simply keeping the architecture the same in the instance.