BiDi: Overall RTL mode
This is probably for the far future, when utilities and distributions catch up so much with RTL that it can be made the default mode for RTL users, at least in some distributions specifically targeted at such markets (e.g. the shell prompt would also consist of text that goes from right to left, etc.).
In overall RTL mode:
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the default mode is RTL;
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resetting, as well as the SCP 0 escape sequence switches to RTL (SCP 1 and 2 remain for LTR and RTL, respectively);
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the extra gap (if the window's width is not grid-aligned) would go to the left.
It's unclear to me whether it's better to have a standalone API for this, or rely on the widget's GtkTextDirection. It's also unclear whether it should be settable runtime, or only at widget construction.