Add textbox indexes, update trinket/crew textboxes

This adds an attribute to textboxclass to allow a text box to keep an
index that references another text box inside the graphics.textboxes
std::vector.

This is needed because the second text box of a "You have found a
shiny trinket!" or "You have found a lost crewmate!" pair of text boxes
explicitly relies on the height of the first text box. With this, I have
moved those text boxes over to the new text box translation system.

Since the update order now matters, I added a comment to
recomputetextboxes() that clarifies that the text boxes must be updated
in linear order, starting from 0.

desktop_version/src/Graphics.cpp

@@ -1525,6 +1525,17 @@ int Graphics::getlinegap(void)
     return 1;
 }
 
+void Graphics::textboxindex(const int index)
+{
+    if (!INBOUNDS_VEC(m, textboxes))
+    {
+        vlog_error("textboxindex() out-of-bounds!");
+        return;
+    }
+
+    textboxes[m].other_textbox_index = index;
+}
+
 void Graphics::createtextboxreal(
     const std::string& t,
     int xp, int yp,