Introduce WindowSystem

[lenemter]

Reducing diff of #361

Changes:

1. AppWindow -> Window since this object stores info about opened window, it is not AppSystem specific.
2. Introduces WindowSystem that communicates with DesktopIntegration and syncs internal list of Window objects. Then AppSystem (and WorkspaceSystem in the future) can use this list to update apps' list of windows etc.

[alainm23]

I'm not sure what the best Git workflow would be in this case, but at my workplace, for a new feature, we create a new branch derived from main, for example, feat/workspace-switcher. We then submit PRs little by little to that branch. Once everything is okay and tested, we submit a PR to main. I mention this because you might make more adjustments and changes for this new feature. What do you think @danirabbit?

[danirabbit]

Personally I prefer this way of merging in smaller more reviewable chunks. It's easier on reviewers and I think we're more likely to catch things as we go than trying to merge in one huge branch