Scroll Wheel Support

The scroll wheel now properly scrolls text within the chat windows!
ChatMode

Added a new enum and graphic that show which type of chat you're
currently doing, called ChatMode. This displays the icon 'public',
'private', 'global', 'group', etc. based on the type of chat that you're
doing.
Private chat shortcut +scroll bugfix

Added shortcut for the ! shorthand notation when the current selected
tab is one of the private chat windows and you don't type the character
name to PM them. Also fixed bug with scroll bars not working for the
other chat tabs because their parent is not the XNAPanel.
Right-clicking PMs

Right clicking a line of text in the chat window will put their name in
the chat text box to start a pm conversation. Also refactored the code
in HUD so that the button click event handlers are lambdas instead of
being individual private methods.