Add textimage for levelcomplete and gamecomplete

`levelcomplete` and `gamecomplete` were hardcoded using textbox colors
which were offset by 1. This PR fixes that, no longer requiring
slightly-off colors, and instead adding a new property to textboxes
which tell the game to display either level complete or game complete.

desktop_version/src/Textbox.cpp

@@ -30,6 +30,8 @@ textboxclass::textboxclass(void)
     fill_buttons = false;
 
     sprites.clear();
+
+    image = TEXTIMAGE_NONE;
 }
 
 void textboxclass::addsprite(int x, int y, int tile, int col)
@@ -42,6 +44,11 @@ void textboxclass::addsprite(int x, int y, int tile, int col)
     sprites.push_back(sprite);
 }
 
+void textboxclass::setimage(TextboxImage new_image)
+{
+    image = new_image;
+}
+
 void textboxclass::centerx(void)
 {
     resize();