Remove textboxclass's firstcreate and clear, simplify initializations

As a result of the previous commit, textboxclass::clear() is now unused.
textboxclass::firstcreate() was already useless. So remove both those
functions and initialize the values in the textboxclass constructor.

desktop_version/src/Textbox.h

@@ -10,10 +10,6 @@ class textboxclass
 public:
     textboxclass();
 
-    void firstcreate();
-
-    void clear();
-
     void centerx();
 
     void centery();