Factor out the game toolbars into a class.

This includes the following related or incidental changes:
* Remove the win_from_rects global variable. With one minor exception, they were all equal to the relevant render texture's size anyway.
* Split out time advancement from the handle_action function into a separate function
* Split out each individual button action into its own function
* Thanks to the above two, button actions triggered from the keyboard (and menu spells) no longer pass thru handle_action
* Side-effect: keyboard shortcuts and menu spells no longer trigger the button press animation
* Button presses now behave like proper buttons
* Button clicks are now grouped by effect in the handling code, rather than by index
* Removed a variable that mysteriously caused dialogue to become blank

src/game/boe.graphics.hpp

@@ -24,13 +24,11 @@ void draw_startup(short but_type);
 void draw_anim();
 void place_anim(); 
 void draw_start_button(eStartButton which_position,short which_button);
-void main_button_click(int which_button);
 void arrow_button_click(rectangle button_rect);
 void end_startup();
 void load_main_screen();
 void redraw_screen(int refresh);
 void put_background();
-void draw_buttons(short mode);
 void draw_text_bar();
 void refresh_text_bar();
 void put_text_bar(std::string str);