gui2/ttoggle_button et al: Make GUI__BACKGROUND_COLOR_DISABLED transparent

Affects ttoggle_button "listbox_text" and "listbox_text_with_icon", and
ttoggle_panel.
gui2/ttext_box: Use standard GUI__BACKGROUND_COLOR_* background colors

Note that this widget lacked a background before, making it look like
a simple label with a border and caret. In my opinion, the subtle
background is an improvement that highlights the widget's "dynamicness",
and makes it more obvious when it is disabled.