Make the Whisper button put /whisper @name at front of text

Currently, using the Whisper button from the popup menus simply adds the /whisper part wherever the text cursor is currently. This causes the whisper to fail if a user has already typed out a message (leaving the cursor at the end).

Having the Whisper button add the /whisper part at the start of the text input field (as the first character onwards) would prevent this.