This PR adds support for (optionally) displaying a popup message to the user when first launching the application. Specifically, with this change, Gridsync will check for a [message] section in config.txt when starting up and, if it is found, display a QMessageBox to the user with the title, text, and (icon) type specified by that section (where type is one of "information", "warning", or "critical") -- for example:

[message]
type = information
title = Cat Fact 8413 
text = Cats have been domesticated for around 4,000 years

Silly example aside, the purpose of this change is to allow whitelabel/rebranded distributions of Gridsync to display custom messages/reminders to their users (for example, about where to obtain an invite code or about who to contact for further help/support). Because this feature can constitute an annoyance, however, users can choose to suppress subsequent messages by checking/ticking the "Do not show this message again" checkbox inside the initial message box.