Results 6 issues of Rene Hexel

This adds an example in Swift. The implementation uses idiomatic Swift, largely resembling the existing examples in naming and structure.

This upgrades the Swift language version to 4.2 (also tested with Swift 5.1 on macOS and Swift 5.0.1 on a Raspberry Pi).

Now that [SwiftGObject](https://github.com/rhx/SwiftGObject) has the ability to register swift objects, this should be automated in the generated constructor for any classes that define from `GLibObject.Object`.

Currently `gir2swift` just blindly translates `.gir` file interface implementations into protocol conformances, leading to redundant conformance warnings, e.g.: ``` Gtk-3.0-TearoffMenuItem-UIManager.swift:25726:128: warning: redundant conformance constraint 'Self': 'TreeModelProtocol' public protocol TreeStoreProtocol: GLibObject.ObjectProtocol,...

When compiling SwiftGtk, numerous labels show up as unnecessary duplicates, e.g.: ``` Gtk-3.0-PadControllerClass-RadioActionClass.swift:1821:53: warning: extraneous duplicate parameter name; 'keyFile' already has an argument label @inlinable init(keyFile keyFile: KeyFileT, groupName: UnsafePointer?...

This package doesn't actually require Swift 5.2 (it builds and runs fine with Swift 5.1 except for the `swift-tools-version:5.2` declaration in `Package.swift`) -- this pull request enables pulling in and...