Toolbars for iOS apps' MainWindows.

[johnzhou721]

Adds toolbar support to iOS MainWindows.

All caveats and user cautions documented inline.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[freakboy3742]

Whenever I'm reviewing a PR, my first question is "How do I test this?". I can confirm that the Command example does work; but with a few weird glitches - which possibly fall inside the boundaries of the platform notes you've documented

But that's pointing at the bigger issue here - you've implemented a bunch of logic, making a bunch of design decisions, without actually confirming if those design decisions are appropriate or acceptable. The fact that you've got a multiple paragraphs of platform notes, and no other platform requires those notes should be an indicator of something.

This is a non-trivial body of work, being where there isn't a straightforward "implement the same API on a new platform". The first step is to propose a course of action, not implement it. An implementation could be a useful proof of concept - but only if that's the easiest path to illustrate the proposal.

So - in this case, I suggest stepping back a bit and at least documenting the concerns here so we can have a conversation about the design decisions you've made.

[johnzhou721]

Given that toolbars have nontrivial interactions with Scaffolds, which I'm already working on, I am going to close this PR; however, for the sake of posterity, the icons becoming monocolor issue can be alleviated by the use of https://developer.apple.com/documentation/uikit/uiimage/withrenderingmode(_:)?language=objc ; whether we should render colors in monocolor to match system is another topic not suitable for discussion on this PR.