Add Switches, Fire/Water Collision and Play Menu

* New doodads: Switches.
  * They come in four varieties: wall switch (background element, with
    "ON/OFF" text) and three side-profile switches for the floor, left
    or right walls.
  * On collision with the player, they flip their state from "OFF" to
    "ON" or vice versa. If the player walks away and then collides
    again, the switch flips again.
  * Can be used to open/close Electric Doors when turned on/off. Their
    default state is "off"
  * If a switch receives a power signal from another linked switch, it
    sets its own state to match. So, two "on/off" switches that are
    connected to a door AND to each other will both flip on/off when one
    of them flips.
* Update the Level Collision logic to support Decoration, Fire and Water
  pixel collisions.
  * Previously, ALL pixels in the level were acting as though solid.
  * Non-solid pixels don't count for collision detection, but their
    attributes (fire and water) are collected and returned.
* Updated the MenuScene to support loading a map file in Play Mode
  instead of Edit Mode. Updated the title screen menu to add a button
  for playing levels instead of editing them.
* Wrote some documentation.

color.go

@@ -211,6 +211,11 @@ func (c Color) DecodeMsgpack(dec *msgpack.Decoder) error {
 // 	return nil
 // }
 
+// IsZero returns if the color is all zeroes (invisible).
+func (c Color) IsZero() bool {
+	return c.Red+c.Green+c.Blue+c.Alpha == 0
+}
+
 // Add a relative color value to the color.
 func (c Color) Add(r, g, b, a int) Color {
 	var (
@@ -237,6 +242,16 @@ func (c Color) Add(r, g, b, a int) Color {
 	}
 }
 
+// AddColor adds another Color to your Color.
+func (c Color) AddColor(other Color) Color {
+	return c.Add(
+		int(other.Red),
+		int(other.Green),
+		int(other.Blue),
+		int(other.Alpha),
+	)
+}
+
 // Lighten a color value.
 func (c Color) Lighten(v int) Color {
 	return c.Add(v, v, v, 0)