A widget's style (font) is only set after the widget has been
packed and inherits relevant information from the top-level window.
`set_size_request_to_display_given_text()` produces incorrect
results when it is called for widgets without a top-level
(here ARDOUR_UI::setup_windows() _main_window.add(), changes the
font from "Sans 11" to "Sans 9" for ArdourDropdowns).
`set_size_request_to_display_given_text()` must subscribe to
`signal_style_changed()`, however in case of ArdourButton UI-scaling
of elements also needs to be taken into account. It is preferable
to allow for multiple sizing-texts directly.
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