Slight changes for new command-prefix hiding feature:

Moved logic checking input to determine if command prefix should be
re-added out of GuiChatTC.keyTyped and into
TabbyChatUtils.writeLargeChat to make better use of already-present
check on active tabs

Added logic to GuiChatTC.checkCommandPrefixChange to NOT automatically
insert the command prefix in the input box when activating a tab with a
defined prefix IF the hidePrefix flag is set.