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Passing either WIN_LEFTKEY or WIN_RIGHTKEY in the window "mode" argument enables this new indicator in the upper right of "list" windows. These indicator arrows are mouse-clickable too (translate to the left and right arrow keys).

We had defined uses for all 32 available WIN_* mode bits, so I extended the win mode type from 32 to 64 bits (should this now be called uifc64.c?) :-)

Anyway, it'll be easier to add more WIN_* mode flags as needed later, but I do wonder about JavaScript support (not so great for > 31-bit integers).