[GUI] add [Mark as cut point] functionality,shortcut,button and menu entry

[szlldm]

We can add cut point without deleting actual frames (== splitting the segment into two).
This can be very useful for example marking scene changes for partial filtering.

[eumagga0x2a]

I might profoundly misunderstand the design of Avidemux, but currently it looks for me like all the segment stuff is on purpose internal to the editor and never leaks into the filter chain.

Semantically, a cut point is a discontinuity. Do you try to address the problem of missing multiple selection capability by other means?

In any case, I would suggest delaying this topic until the next release cycle.

[szlldm]

In Copy mode it simply show an error message. After release a segment independent solution can be figured out.
This function is heavily required.