allow Shift+Backspace to act as Backspace in insert mode

src/gui/Window.cc

@@ -552,6 +552,7 @@ uint32_t Window::get_shifted(uint32_t keysym)
     case SDLK_RIGHTBRACKET: return '}';
     case SDLK_BACKQUOTE:    return '~';
     case SDLK_SPACE:        return ' ';
+    case SDLK_BACKSPACE:    return '\b';
     }
 
     return 0u;