Add menu items and shortcuts to switch pasteboards

[sindresorhus]

Command+1, Command+2, etc.

I tried putting

Pasteboard-Viewer/Pasteboard Viewer/ContentView.swift

Line 80 in a41051b

EnumPicker("Pasteboard", enumBinding: $selectedPasteboard) { pasteboard, _ in

inside the menu, but macOS 11 doesn't currently have a working way to make the menu items only enabled when the window is focused. This can wait until macOS 12.