Add server list to `DeltaClientGtk` (the UI for non-Apple platforms)

[stackotter]

Context

There is an experimental DeltaClient UI called DeltaClientGtk which is built off SwiftCrossUI. The intention of this subproject is to port DeltaClient to non-Apple platforms (initially just Linux). This UI only supports connecting directly to servers by typing in their address, and interacting with the server via chat. Creating a cross platform world renderer is waiting for Kinzoku or another similar project to become feature complete enough to implement a full performant renderer.

Adding a server list

It would be very useful to be able to add your favourite servers to a server list (just like you can in regular DeltaClient). You can use the Split example from the SwiftCrossUI repo as a reference for implementing a sidebar similar to that used by regular DeltaClient.

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