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/Textbox.cpp

@@ -45,6 +45,8 @@ textboxclass::textboxclass(int gap)
     original = TextboxOriginalContext();
     original.text_case = 1;
     spacing = TextboxSpacing();
+
+    other_textbox_index = -1;
 }
 
 void textboxclass::addsprite(int x, int y, int tile, int col)