Correct undefined background Main Tab

Correct the background of the world editor main tab with a (new) solid fill. Sometimes it appeared as black, magenta or the just the scene itself. Color is fixed for now as it is a unique situation regarding the use of tabs here.

Templates/BaseGame/game/tools/gui/profiles.ed.tscript

@@ -943,6 +943,14 @@ singleton GuiControlProfile( GuiBackFillProfile )
    category = "Editor";
 };
 
+singleton GuiControlProfile( GuiDarkFillProfile )
+{
+   opaque = true;
+   fillColor = "25 25 25";
+   border = false;
+   category = "Editor";
+};
+
 singleton GuiControlProfile(GuiShaderEditorProfile : ToolsGuiDefaultProfile)
 {
    opaque = true;

Templates/BaseGame/game/tools/worldEditor/gui/EditorGui.ed.gui

@@ -19,6 +19,19 @@ $guiContent = new GuiContainer(EditorGui,EditorGuiGroup) {
    AnchorLeft = "1";
    AnchorRight = "0";
    
+   new GuiDecoyCtrl(EditorGuiMainTabBookBG) {
+      profile = "GuiDarkFillProfile";
+      horizSizing = "width";
+      vertSizing = "bottom";
+      position = "0 5";
+      extent = "800 24";
+      minExtent = "8 8";
+      visible = "1";
+      helpTag = "0";
+      useMouseEvents = "0";
+      isDecoy = "1";
+   };
+   
    new GuiTabBookCtrl(EditorGuiMainTabBook) {
       tabHeight = "20";
       selectedPage = "0";