TTT: Added Scoreboard hooks

Added hooks:
"TTTScoreboardColumns" - Two arguments, Panel and Enumeration. Return nil.
"TTTScoreboardMenu" - One argument, Panel. Return PreventOpen (Boolean).

Added enumerations for TTTScoreboardColumns hook:
TTT_COLUMN_HEADING - Column headings
TTT_COLUMN_ROW - Column row/data
TTT_COLUMN_BACKGROUND - Column background (Darker columns)

Added functions PANEL:AddColumn( label, func ) to scoreboard panels.