Allow to implement non-node-aware SelectEditor DataSources

[skurfuerst]

currently, the data sources for the SelectEditor always get the current node passed in as parameter. However, many data sources do not depend on the current node at all. If we had this information in the UI, we could optimize the loading of data sources to cache them more aggressively; leading to a better performance when using many data sources and switching nodes.

Implementation Idea

• I'd suggest to add an additional interface on the PHP side. This is a little more difficult to implement (as this info is not existing in the UI right from the start), but is a lot cleaner than introducing another data source parameter I think.
• Using Reflection (and compileStatic), we fetch this information, and send it as part of the config to the UI. Alternatively, we could send it as part of the first response.

Related idea

• Add DataSources which can do server-side filtering, e.g. sending the search term to the server!

Timeframe

This is all post-1.0-relevant I think. I just wanted to write it down while working on #764

[dimaip]

@skurfuerst is this something you still want to do/make sense of doing? If not I'd close the issue.