UI Component Modularization/Abstraction

[hardl]

Luigi core comes with built-in own UI controls based on fundamental styles, e.g. shellbar, left nav, modals, etc.
So far, our customization strategy was to override Fiori CSS for non-Fiori layouts, which starts to get hard to maintain due to a constantly growing functional scope, both, on luigi side and on consumer side. In addition, the luigi core bundle size is increasing, even though a lot of functionality might not be needed in a custom layout scenario.

This epic covers activities to make luigi core modular wrt UI controls, i.e. replacing built-in UI code by a headless abstraction layer which then can be developed against in custom UI scenarios. In order to still provide an easy start with luigi, the current UI functionality should be provided as a "pluggable" bundle, ideally using existing UI controls wherever possible (e.g. ui5 web components) to lower TCO.

Issue in this epic:
#3233
#3782
#3776