Implement UI for a server list in DeltaClientGtk

[ninjadev64]

Description

This PR implements UI for a server list in DeltaClientGtk. It may be followed with another PR that implements actually storing servers, but as of this PR they are not persisted.

Fixes #183

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation comments
• [x] My changes generate no new warnings

[ninjadev64]

All done :+1:

[ninjadev64]

I'll add removing servers later today so don't merge yet

[ninjadev64]

Actually nvm, I'll add removing and editing as part of my next PR - feel free to merge.