Add missing layout widgets

[cmyr]

There are a number of basic layout widgets in flutter that we don't have.

• : A widget that checks if incoming constraints are infinite, and constraints them.
• : A widget that fits a child within itself, in a number of ways (fill, fit, fillHeight, etc).
• : use a fraction of available space
• x: maintain an aspect ratio (this should report if it is passed tight constraints) #1645
• : allow a child to overflow
• : impose arbitrary(?) constraints on child? I think this should probably be represented as Fn(BoxConstraints) -> BoxConstraints or something.

In any case: none of these are super-duper-urgent, but I think we would like to have all of them eventually.

[kindlychung]

• [ ] Rotate: allows rotation of its child.

[JAicewizard]

just a question, but how is drawing outside the window border being handled? would the OverflowBox clip the content on the edge of the window? or would it signal the window to grow somehow. If the later is chosen, will other widgets resize too? will the background be painted on the part of the window that wasn't there before. If the OverflowBox would grow the window without resizing or painting the background on the new area, it might be nice to use it for dropdowns, since they might not fit withing the window.

[cmyr]

Outside of the window or outside of the widget?

I do not expect us to support drawing outside of the window; for cases like drop-down menus, we will be creating new modal windows for the menu.