The Replace Clip command doesn't replace the proper in/out length properties for the new clip

It was done like that as in most cases the replacement clip will be a different length, but we can probably enhance it to check if the length is the same, and then it that case use the existing start/end points.

This patch fixes that. If the file length is the same for both the file of the clip that is getting replaced and the new file, it uses the same in and out points.
It also fixes two other minor issues:
1) If a folder was right clicked, a separator was shown at the bottom of the menu.
2) If something that wasn't a video clip was right clicked on the timeline, double separators were shown in the menu.

Thanks a lot Emil not only for this one but for all your work.

Yes, thanks for your work Emil. The world need more people like you.

Thanks, this has been applied to the trunk. I also noticed the thumbnail wasn't getting updated on the replacement clip, so I fixed that too.