Event System (#1741)

* Add implementation of the EventEngine to handle global event chains
* Add properties to SceneGraphNodes to determine two distance radii for camera-based events

data/assets/examples/approachevents.asset

@@ -0,0 +1,68 @@
+local assetHelper = asset.require('util/asset_helper')
+local sunTransforms = asset.require('scene/solarsystem/sun/transforms')
+local transforms = asset.require('scene/solarsystem/planets/earth/transforms')
+
+local generic_action = {
+  Identifier = "os.example.generic",
+  Name = "Generic Example",
+  Command = [[
+    openspace.printInfo("Node: " .. args.Node)
+    openspace.printInfo("Transition: " .. args.Transition)
+  ]],
+  Documentation = "Prints the argument information for camera transitions to the log",
+  GuiPath = "/Examples/Events",
+  IsLocal = true
+}
+
+local model = asset.syncedResource({
+  Name = "Animated Box",
+  Type = "HttpSynchronization",
+  Identifier = "animated_box",
+  Version = 1
+})
+
+local obj = {
+  Identifier = "ExampleEventModel",
+  Parent = transforms.EarthCenter.Identifier,
+  Transform = {
+    Translation = {
+      Type = "StaticTranslation",
+      Position = { 0.0, 11E7, 0.0 }
+    }
+  },
+  Renderable = {
+    Type = "RenderableModel",
+    GeometryFile = model .. "/BoxAnimated.glb",
+    ModelScale = 1.0,
+    LightSources = {
+      {
+        Type = "SceneGraphLightSource",
+        Identifier = "Sun",
+        Node = sunTransforms.SolarSystemBarycenter.Identifier,
+        Intensity = 1.0
+      }
+    },
+    PerformShading = true,
+    DisableFaceCulling = true
+  },
+  InteractionSphere = 1000.0,
+  OnApproach = { "os.example.generic" },
+  OnReach = { "os.example.generic" },
+  OnRecede = { "os.example.generic" },
+  OnExit = { "os.example.generic" },  
+  GUI = {
+    Name = "Example Event Model",
+    Path = "/Example",
+    Description = "",
+  }
+}
+
+asset.onInitialize(function()
+  openspace.action.registerAction(generic_action)
+  openspace.addSceneGraphNode(obj)
+end)
+
+asset.onDeinitialize(function()
+  openspace.removeSceneGraphNode(obj.Identifier)
+  openspace.action.removeAction(generic_action.Identifier)
+end)