Drag and drop: Standardizing payload types as defines. (#143)

imgui_internal.h

@@ -167,6 +167,10 @@ inline void operator delete(void*, ImPlacementNewDummy, void*) {}
 // Types
 //-----------------------------------------------------------------------------
 
+// Drag and Drop payload types. String starting with '_' are managed by Dear ImGui.
+#define IMGUI_PAYLOAD_TYPE_COLOR_4F         "_COL4F"    // float[4]     // Standard type for colors. User code may use this type. Build a float[4] out of a float[3] if you don't have alpha.
+#define IMGUI_PAYLOAD_TYPE_DOCKABLE         "_IMDOCK"   // ImGuiWindow* // [Internal] Docking/tabs
+
 enum ImGuiButtonFlags_
 {
     ImGuiButtonFlags_Repeat                 = 1 << 0,   // hold to repeat
@@ -239,7 +243,8 @@ enum ImGuiDir
     ImGuiDir_Left    = 0,
     ImGuiDir_Right   = 1,
     ImGuiDir_Up      = 2,
-    ImGuiDir_Down    = 3
+    ImGuiDir_Down    = 3,
+    ImGuiDir_Count_
 };
 
 enum ImGuiCorner