gui2/ttoggle_button et al: Use transparent color background to match GUI1

Replace the definition of GUI__BACKGROUND_COLOR_ENABLED (used by the
"listbox_text" and "listbox_text_with_icon" variations of
ttoggle_button, and the default ttoggle_panel variation) so that it
matches the gui::menu::bluebg_style GUI1 menu style definition in
src/widgets/menu_style.cpp:29.

These controls are commonly used with gui2::tlistbox, so this change is
intended to make GUI2 listboxes in general have the same overall
appearance as GUI1 menus after the dialog background graphics were
replaced as part of the UI overhaul in 1.11.x.

The previous background definition for these GUI2 widgets was a solid
color, rgb(21, 19, 19), meaning that they necessarily wouldn't match
the standard bluebg_style GUI1 menu style, which uses rgba(0, 0, 0,
0.35).

Alpha 255 * 0.35 = 89.25, hence the new definition of
GUI__BACKGROUND_COLOR_ENABLED has an alpha channel value of 89 and not a
multiply of 10 or 2 like 90.