Add visual targeting.

[tamtale]

• When selecting a group of minions, any entity targeted by one of the minions will display a red "X".
• This feature is dictated by the "visualTargeting" preference in the preferences file (off by default).
• Migrated edge panning to use prefs for consistency.
• added convenience publish and getSubscribers method to Publisher.
• Cursor now only changes according to clickable if it's visible (added method to interface).
• Towers are now Clickabe, with the expected highlighting (Opponent towers not being highlighted fixed in #155)
• Moved anonymous Block Clickable class to an actual class