add built-in OSD message for command `frame-step` and `frame-back-step` to show frame number

Expected behavior of the wanted feature

I think frame number is an important message, it's useful for compare scaling filters/shaders, open same file in two window and ensure that they are exactly in the same frame.

Try to add OSD message by myself, but the show-text shows the frame number before frame-step and frame-back-step.
e.g.
frame-step from 0 to 1, show-text shows 0 / 12345 not 1 / 12345.
frame-back-step from 1 back to 0, show-text shows 1 / 12345 not 0 / 12345.

KEY1   frame-step; show-text "${estimated-frame-number} / ${estimated-frame-count}"
KEy2   frame-back-step; show-text "${estimated-frame-number} / ${estimated-frame-count}"

So a built-in OSD message is needed, if some people don't like it, add no-osd prefix to disable it.

Alternative behavior of the wanted feature

N/A

Log File

N/A

Sample Files

N/A

Start by tracing the frame-step and frame-back-step command handlers and the existing show-text OSD message path. Check how estimated frame values are evaluated relative to command execution, then verify that both commands display the resulting frame number and that the proposed no-osd behavior is covered.