Add "About GPG Sync" window with installed version number

[huertanix]

When asking colleagues what version of GPG Sync they are running, they should ideally be able to navigate to a GPG Sync -> About window and see a logo with the version and maybe build number, as is common in other macOS applications.

[micahflee]

Oh that's a good idea. Right now you can tell by clicking the systray icon in the top-right, the version is the first item in the context menu.