Add support for images in dialogue windows (#19)

- Extend editor nodes with custom DialogueContent data type that holds either image or text
- Extend the dialogue importer to correctly process the new content into updated RuntimeDialogue content
- Update SpeechBubble to be able to display either text or image
- Add a custom property drawer for the DialogueContent to allow easy switching in graph authoring

Co-authored-by: Michal Pikulski <michal@foolhardyhorizons.com>
Reviewed-on: #19

Assets/Dialogue/Gardener/GardenerDialogueGraph.dialoguegraph

@@ -19,6 +19,20 @@ MonoBehaviour:
     RefIds:
     - rid: -2
       type: {class: , ns: , asm: }
+    - rid: 7545629632211976320
+      type: {class: 'Constant`1[[Dialogue.DialogueContent, AppleHillsScripts]]', ns: Unity.GraphToolkit.Editor, asm: Unity.GraphToolkit.Internal.Editor}
+      data:
+        m_Value:
+          _contentType: 0
+          _text: 
+          _image: {fileID: 0}
+    - rid: 7545629632211976321
+      type: {class: 'Constant`1[[Dialogue.DialogueContent, AppleHillsScripts]]', ns: Unity.GraphToolkit.Editor, asm: Unity.GraphToolkit.Internal.Editor}
+      data:
+        m_Value:
+          _contentType: 0
+          _text: 
+          _image: {fileID: 0}
     - rid: 7772910664224079994
       type: {class: GraphModelImp, ns: Unity.GraphToolkit.Editor.Implementation, asm: Unity.GraphToolkit.Editor}
       data:
@@ -296,15 +310,15 @@ MonoBehaviour:
           m_KeyList:
           - __option_DialogueLineType
           - __option_NoLines
-          - DefaultDialogueLine2
           - LoopThroughDefaultLines
-          - DefaultDialogueLine1
+          - DefaultDialogueContent1
+          - DefaultDialogueContent2
           m_ValueList:
           - rid: 7772910664224080006
           - rid: 7772910664224080007
-          - rid: 7772910664224080012
           - rid: 7772910664224080009
-          - rid: 7772910664224080011
+          - rid: 7545629632211976320
+          - rid: 7545629632211976321
         m_InputPortInfos:
           expandedPortsById:
             m_KeyList: []
@@ -374,14 +388,6 @@ MonoBehaviour:
     - rid: 7772910664224080010
       type: {class: DialogueNode, ns: Editor.Dialogue, asm: AppleHillsEditor}
       data: 
-    - rid: 7772910664224080011
-      type: {class: 'Constant`1[[System.String, mscorlib]]', ns: Unity.GraphToolkit.Editor, asm: Unity.GraphToolkit.Internal.Editor}
-      data:
-        m_Value: My lawnmower is on a break so i'm also on a break...
-    - rid: 7772910664224080012
-      type: {class: 'Constant`1[[System.String, mscorlib]]', ns: Unity.GraphToolkit.Editor, asm: Unity.GraphToolkit.Internal.Editor}
-      data:
-        m_Value: I ain't moving until she decides to move...
     - rid: 7772910664224080013
       type: {class: UserNodeModelImp, ns: Unity.GraphToolkit.Editor.Implementation, asm: Unity.GraphToolkit.Editor}
       data: