[Core] Update: Request for name at client join.

Produced offline, so requires testing.

Testing method:
1. Join server
2. Frame should appear with a text box (yes, it's ugly, that's not what needs testing!)
2a. If the frame does not pop up, then it's not working.
3. Type in "[This name should error!]" and press enter
3a. A message should pop up saying invalid name for about a second, if this does not occur, then it's not working.
3b. If the frame closes after pushing enter after entering this, then it's not working.
4. Type in "This name is okay" and press enter
4a. The frame should close, if this does not occur, then it's not working.
4b. If it states an invalid name was given, check the input only contained spaces and letters.
4ba. If it continues to state an invalid name was given, then it's not working.