Fix logic for .globalKeyboardShortcut()

[cweider]

Evaluating globalKeyboardShortcut(_:) revealed several issues with related to special keys:

• Shortcuts using the Key.space would produce a KeyEquivalent that specified the "S" key. (cause: first being used on the keyToCharacter() return value, "space")
• Shortcuts using Key.f* would produce a KeyEquivalent that specified the "F" key. (cause: same as above)
• Shortcuts using Key.keypad* would produce a KeyEquivalent that was not limited to the number pad. (cause: neither SwiftUI nor AppKit make this distinction when producing menu items)

Considering these issues, a reevaluation of special keys' handling is called for. Excepting the Carbon integration that drives the actual keyboard shortcuts, there are three distinct contexts where a Shortcut is used:

• Recoder: where an NSString is produced to go into the TextField displaying the shortcut's current value
• NSMenuItem: for specifying keyEquivalent and keyEquivalentModifierMask
• toSwiftUI: for creating a SwiftUI.KeyboardShortcut

Ideally, each of these three do their work in analogous ways. This branch accomplishes this with the following:

• SpecialKey enumerates the cases that require extra attention. This is drawn directly from keys used in keyToCharacterMapping and used to ensure that each special case is handled.
• Three mappings for SpecialKey are created for each of the three contexts where the handling is important: presentableDescription, swiftUIKeyEquivalent, and appKitMenuItemKeyEquivalent.
• In these three contexts if there is a SpecialKey, the defined mapping is what rules. keyToCharacter() is used if and only if there is no SpecialKey available. An assertion to this effect is added.

Fixes: #192

[cweider]

For testing, this is a snippet that you can go and stick any old place:

Menu("Menu", systemImage: "filemenu.and.selection") {
	Button("Shortcut 1") {
		// no-op
	}
	.globalKeyboardShortcut(.testShortcut1)

	Button("Shortcut 2") {
		// no-op
	}
	.globalKeyboardShortcut(.testShortcut2)

	Button("Shortcut 3") {
		// no-op
	}
	.globalKeyboardShortcut(.testShortcut3)

	Button("Shortcut 4") {
		// no-op
	}
	.globalKeyboardShortcut(.testShortcut4)
}

[cweider]

Actual	Expected

[cweider]

I should add that cweider/KeyboardShortcuts@swiftui-key-equivalent is the minimal change for SwiftUI. That this, keyEquivalent, and description each took different approaches to the similar problem motivated this patch.

[sindresorhus]

This is looking very good. Thanks for working fixing this 🙏