Adding back_footer action to general_ui.sh for submenus

When I first ran the code, I went to advanced because I had klipper already installed and I wanted to see if I could install fluidd using this code base.
So when see advanced menu I was concerned that "quit" meant quitting the tool instead of exiting the advanced menu. So proposing a back_footer for sub menus that will revert back to main menu. Save quit footer for main menu to exit program.

scripts/ui/general_ui.sh

@@ -21,6 +21,12 @@ quit_footer(){
   bottom_border
 }
 
+back_footer(){
+  hr
+  echo -e "|                        ${green}B) Back${default}                        | "
+  bottom_border
+}
+
 print_header(){
   top_border
   echo -e "|     $(title_msg "~~~~~~~~~~~~~~~~~ [ KIAUH ] ~~~~~~~~~~~~~~~~~")     |"