Replace chat line with UI

[no-lex]

The console/chat line in the engine is hardcoded and hard to modify. Unlike the rest of the user interface, which is created using the ui.cpp ui system, the chat line is a philistine, coded in native C++. Making this part of the UI like other parts of the UI will help to make the whole system more consistent and understandable, and allow for better UI implementation of the console/chat input itself.