popn: implement bi2x hook (#623)

## Link to GitHub Issue or related Pull Request, if one exists
#618 

## Description of change
Implement BI2X hook I/O for service/test/coin/headphones and the deka
pop-kuns.

src/spice2x/games/popn/io.cpp

@@ -11,6 +11,7 @@ std::vector<Button> &games::popn::get_buttons() {
                 "Service",
                 "Test",
                 "Coin Mech",
+                "Headphones",
                 "Button 1",
                 "Button 2",
                 "Button 3",
@@ -19,7 +20,9 @@ std::vector<Button> &games::popn::get_buttons() {
                 "Button 6",
                 "Button 7",
                 "Button 8",
-                "Button 9"
+                "Button 9",
+                "Red Pop-Kun",
+                "Blue Pop-Kun"
         );
     }
 
@@ -29,8 +32,8 @@ std::vector<Button> &games::popn::get_buttons() {
 std::string games::popn::get_buttons_help() {
     // keep to max 100 characters wide
     return
-        "  2 4 6 8  \n"
-        " 1 3 5 7 9 "
+        "RED   2 4 6 8   BLUE\n"
+        "     1 3 5 7 9"
         ;
 }