Added empty state messages in breadcrumb selectors (Header)

[ShailRT]

Description

This PR fixes #4099 and displays a message when there are no items in breadcrumb selector.

Database selector:

• When there are no databases, it display a "No Databases configured" text in the dropdown.
• We display a peristent link to "Connect a database".

Schema selector:

• When there are no schemas, we display a "No Schemas configured" text in the dropdown.

Table selector:

• When there are no tables, we display a "No Tables configured" text in the dropdown.

Screenshot

Before (No Database):

After (No Database):

Before (No Schema):

After (No Schema):

Before (No Table/Exploration):

After (No Table/Exploration):

Checklist

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• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[seancolsen]

Well this ended up being more involved that I would have predicted. Due to the way that BreadcrumbSelector is abstracted, it's actually not straightforward to just stick some UI in there like "No Databases configured" and do it in an i18n-friendly way. This PR had {`No ${categoryName} configured`} in the Svelte template. Sorry, but we can't do that because all our UI strings need to be run through translation. And we need to translate entire sentences at a time in order to properly deal with all the complexities of different languages.

In fe552ce I heavily refactored this code to improve the structure and to make the no-results messages i18n friendly. You can see where the no-results messages are set if you look at the code modified in DatabaseSelector, SchemaSelector, and EntitySelector. This is much cleaner.

I also removed the "Connect Database" button from inside the dropdown. It would have been more work to implement, and I don't think we need it there.

Even though I've pushed quite a lot of changes here, I'm going to go ahead and merge this myself without an additional review since we're pressed for time. There are quite a lot of subtly different UI states to test here. I could spend 30 minutes producing for review screenshots, but this is a corner that I'm going to cut.

@pavish you might like to glace over the changes here. And try out the result post-merge.