IO: rework interleaved keys<>char trickling: only trickle for keys known to be...

IO: rework interleaved keys<>char trickling: only trickle for keys known to be likely to input characters. (#7889, #4921, #4858)

Amend fa2b318d. Refer to regression test "inputs_io_inputqueue" amended for IMGUI_VERSION_NUM >= 19102