Feat/base structure - Githubissues

This is a usable first version of WindowManager & BaseWindow. Window contents are managed locally, while window visibility is still handled by WindowManager (and the rerendering issue still exists).

Also, grid-layout works as expected now – there was a typo/misconception (?) in the layouts prop of ResponsiveGridLayout.

Also, please don’t close this branch yet, I’m going to implement the rerendering fixes & toolbar here as well because the toolbar will probably be part of the rerendering-issue solution.

Closes #24

Reviewer's task:

[X] I didn't see no code smells
[X] The live preview works as expected -> no regressions found

ISH-Gruppe / screenario

Feat/base structure #22