This PR adds support for optionally specifying a "default" grid/connection via config.txt as follows:

[connection] 
default = demo-grid 

The presence of this option at runtime replaces the "Invite Code" field/widget on the welcome widow with a "Connect to " button which, when clicked, creates a tahoe node using the settings provided by the cheat code specified (thus, in the example above, a "Connect to Demo Grid" button will appear which, when clicked, will load the grid-configuration settings found at gridsync/resources/providers/demo-grid.json).

This (completely optional) setting is intended to be used for custom deployments of Gridsync -- for example, in the event of a user-testing session in which the "invite code" step might need to be skipped, or for commercial deployments that may want to ship "pre-configured" builds to users.