Improve Swift usage in platform_channel_swift some more
Make a bunch of changes that willlarche requested in
https://github.com/flutter/flutter/pull/21712:
* Use `static let` within `enum` namespaces for String constants.
* Use `guard` for early exits and to avoid forced unwrapping.
* Explicitly qualify a selector.
* Fix a retain cycle by using a weak reference.
Make platform_channels_swift use more idiomatic Swift
* Remove semicolons.
* `if (condition)` => `if condition`.
* `Class.init(...)` => `Class(...)`.
* Remove `break` statements from `switch` cases.
* Remove some unnecessary uses of `self`.
Generated.xcconfig is only required at build time for iOS apps. In the
flutter create project template and example apps, Generated.xcconfig was
previously marked as a resource to be bundled into the built app.
This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.
* wip
* wip
* delete main.m
* readd main.m
* update to use new channel api
* update android
* removed android/
* remove debug print
* remomved main.m
* Update year in copyright
* break long line and update name
* mit comments
* update examples/README
* break line
* update README
* update test
