accel_char = playout:SetMarkupWithAccel(text$, length, accel_marker)
Replaces the current text and attribute list.
If accel_marker is nonzero, the given character will mark the
character following it as an accelerator. For example, accel_marker
might be an ampersand or underscore. All characters marked
as an accelerator will receive a #PANGO_UNDERLINE_LOW attribute,
and the first character so marked will be returned in accel_char.
Two accel_marker characters following each other produce a single
literal accel_marker character.
text$)