Make textbuttons() work for transparent text boxes

Misa asked me if this should only work for non-transparent textboxes,
and it shouldn't - that was kind of an oversight.

To make it work for transparent textboxes as well, I made a little
restructuring to avoid duplicating the code - fill_buttons() is now
called textbox_line(), and it replaces the direct accessing of the
textbox lines in the printing loops. The code that checks the width
of the textbox does not need to be copied, since the text box is
naturally not drawn for transparent text boxes.

desktop_version/src/Graphics.h

@@ -160,6 +160,7 @@ public:
     void draw_grid_tile(SDL_Texture* texture, int t, int x, int y, int width, int height, SDL_Color color);
 
     void updatetextboxes(void);
+    const char* textbox_line(char* buffer, size_t buffer_len, size_t textbox_i, size_t line_i);
     void drawgui(void);
 
     void draw_sprite(int x, int y, int t, int r, int g, int b);