UI and MP enhancements

[mvlabat]

Closes #31 and closes #33

TODOs:

• [x] Send a disconnect event before manually closing a server (in case of self-hosting)
• [x] Fix server sending Uninitialized messages, when there's already a Host
• [x] Fix handling messages from previous connections (introduce tracking session ids, reject if they don't match)
• [x] Monitor server process status (for dropping a connection when it crashes)
• [x] Fix client closing a connection
• [x] Kicking
• [x] Sending player name
• [x] Fix OK button transition
• [x] Fix assigning wrong player colors
• [x] Refactor with using UiNetworkCommand
• [x] Fix rejecting players trying to join to a started game