Fix: Remove imports from css and use a list of stylesheets instead

[jperedadnr]

Issue

Fixes #744

Long story short: the @import "com/sun.../modena.css in the css files weren't working (even before the modular changes from #720), as new URL("com.../modena.css)` wasn't being resolved at all.

This also means that CSSInternal::getThemeStyleClasses didn't work at all, because URL("com/....css") is not a valid URL.

This PR moves those CSS imports into code, as a list of different stylesheets used by each theme works (because Javafx-graphics/CSS uses the URLConverter and produces the correct URL(with the right file://javafx.controls.jar!...css path).

Also, CSSInternal::getThemeStyleClasses now uses a class from javafx-controls, so getResource returns a valid URL.

As a result themes are working now:

Progress

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[Oliver-Loeffler]

Looks good to me. I've tested on Windows and it works.