Add healing effect to clearly indicate healing is invoked
Introduce effective range for HealerTower

The tower won't heal others which are farther than the range.
The range will increase as the tower levels up.
Other types of towers, namely offensive towers, may have also attack
range.

The effective range is visualized by overlay tip.
Optimize performance by preventing loading enemy textures repeatedly

Invoking Bitmap.clone() is far faster than recreating Bitmap with
the same texture file name.  This is observed in Firefox, but other
browsers are expected to behave similar way.