Migrate code from aseprite to laf

My plans for the UI part of the laf library are:

- [x] Move the [she library from aseprite](https://github.com/aseprite/aseprite/tree/master/src/she) to `laf::os` namespace.
- [ ] Move the [ui library from aseprite](https://github.com/aseprite/aseprite/tree/master/src/ui) to `laf::ui` namespace.
- [ ] Refactor events (mainly keyboard events) to be more DOM spec-like [[1](https://www.w3.org/TR/DOM-Level-3-Events/)], [[2](https://www.w3.org/TR/DOM-Level-3-Events/#interface-keyboardevent)], [[3](https://www.w3.org/TR/DOM-Level-3-Events/#keys)], [[4](https://cs.chromium.org/chromium/src/ui/events/keycodes/?dr=C)], [[5](https://github.com/WebKit/webkit/blob/dad605136c7145401cdf3917d51894ea5fa7cbdd/Source/WebCore/platform/mac/PlatformEventFactoryMac.mm#L557)].

Start by reviewing the existing move of the she library into the laf::os namespace, then compare aseprite/src/ui with the target laf::ui namespace. Track the remaining UI migration and the DOM-like keyboard event references; done means the UI library is moved and the planned event refactor is complete.

Written by the indexing model from the issue text.