Layout: proportional sizes

[Tiagoperes]

Figure out a way to implement size values based on a fraction of the space available and/or a fraction of the size of the parent.

Our initial idea was to implement something similar to the flex layout, but although this is very simple on Compose, it's very hard on SwiftUI.

Another idea is to implement a component similar to the GeometryReader in SwiftUI.

This issue must be updated as soon as we decide on the actual implementation of this feature.