Consider a mechanism for setting the default namespace in grids

[Ruslan-Aleev]

Feature request

Summary

By default, the "System Settings" and "Lexicons" sections (possibly in other sections) of grids are set to the core namespace.

However, this can cause some problems:

• "What if user doesn't have access to the core namespace?" (as @theboxer rightly pointed out in Added remembering filter properties of lexicons grid in browser url #15942 (comment))
• "It is quite annoying implemented in the system settings grid (at least in 2.8.x): It still shows core there when the search is active but then this filter is not regarded there anymore. And even worse: It shows and filters the first available namespace if the core namespace is on page 2 in the namespace combo. This is normally not visible but on my dev machine I have a lot of namespaces." (comment from @Jako)

Why is it needed?

Avoid possible bugs