Add external libraries development mode.

[dclause]

This PR follows the discussion at #524
The PR is far to be anything but good. It just "POCs" the ideas detailled in the ticket 524.

[falkoschindler]

Yes, using a PR to showcase an idea is a good strategy. 👍
I can see where this is going and I definitely like it. Nevertheless, I'll probably continue without the production flag for now to keep things simple(r) for the upcoming 1.2.0 release. Later in 1.2.1 we should be able apply this PR on top.

Just one thought regarding your current implementation: Is there a reason you moved around the imports? E.g. quasar.css and nicegui.css changed places. And Vue and Quasar JS imports moved down below the #app div. I think we should be careful here because changing the order can lead to quite subtle changes in behavior.

[dclause]

External libraries are usually loaded at the bottom of the body (but before local script code) and deferred. I did not go with defer here, but that could be worth a try.

As of the CSS, I grouped them together in the header. My bad though, I made something stupid: the order should be external libraries, external CSS framework, internal overrides. So nicegui.css should be the last on to load.

[falkoschindler]

In version 1.3.0 the dependency management changed quite a bit, making the proposed changes a bit obsolete. I'll close this PR for now, since it is pretty outdated and stale for months now. But feel free to reopen it or create a new one to continue working on this feature.