Add config option `show_dock_icon`

[tjhorner]

This PR adds a new config option that lets the user control if the icon for the app is shown in the Dock (for macOS only). I found myself always accidentally ⌘+TABing back into the Mstdn window after I closed it by clicking away from the window. If the Dock icon is hidden, the window will not appear in the application switcher, thus preventing that.

By default, it is set to true so existing behavior does not change if the user is updating.

[tjhorner]

I realized just now that if the dock icon is hidden, the menu bar items won't show. ~I will add an option to switch accounts by right-clicking the menubar icon.~ This doesn't appear to be possible with the menubar package.