This PR consists of a number of minor improvements to "invite"-related UI elements and their underlying code, including:

• Wrapping lower-level Wormhole and SetupReceiver methods/signals into higher-level "InviteSender" and "InviteReceiver" QObject classes
• Adding in-app "help" buttons/dialogs on invite-code widgets, explaining "invite codes" and "Tor network" to users
• Fixing some font and UI clipping/overlapping issues observed on macOS and Windows
• Adding "shared" status indicators (overlay icons, tooltips) to remote folders
• Adjusting stylesheets and error dialog text
• Removing the need to restart Gridsync in order to see new grids added via the "Enter Code" dialog
• Displaying service provider icons if/when received via the "Enter Code" dialog
• Prompting for additional user-confirmation when enabling Tor from the welcome screen or "Enter Code" dialogs

Otherwise it gets clipped, for some reason

This fixes overlap issues caused by very low screen resolutions and/or
high global UI scaling settings on Windows