ZKL: fix channel case-insensitiveness & possibly fix channel not really close when closed

1) fix issue reported by mr. Skasi in https://github.com/ZeroK-RTS/Zero-K-Infrastructure/issues/85 ("duplicate channel name with capital letter confuse ZKL"). 
Cause of bug: "Collection" item in NET uses case-insensitive string as key. Manual search fix this.

2) possible fix to issue reported by mr. Licho ("Chat Control not really closed when its told to close, results in error in log"). 
Cause of bug: there was wrong assumption that "panel.Controls" is indexed by string, thus when the Control is to be removed: it fail, and some background code might still be running on that Control causing the error in log.