Clip First frame actual time and Timeline start setting

When you record, for eg press conference, you generally take note about the actual time when an event happens by checking your watch and by writing down what time it is. When you go to edit the content you activate the timecode which represents the time when you recorded the content. The reference to what time it was when you started the recording is stored inside the clip as "first frame timecode" (as you can see in the clip properties by using, for eg, media info.)

It would be very convenient to have the way (as we do when we shift from the timecode to the frames counter) to show this value and the correspondent timecode when you need to use the actual time for editing. This tool exists, of course, in every professional application same as for the fact that you can input a value for the timeline first frame too, so that when you export you can embed this value.

The value on the timeline is used by some broadcaster to export the content so that they are automatically inserted in the right place in a broadcast playlist sequence.

Tasks:

• add a menu entry in the clip monitor, to switch between 'normal timecode' and 'source timecode'

• add somewhere an offset option that would open a dialog allowing to enter a time value that would affect the timecode displayed in timeline and project monitor. Setting first frame timecode for exported clip should be possible through the ffmpeg's timecode parameter: https://video.stackexchange.com/questions/23473/problem-setting-first-frame-timecode-using-ffmpeg-timecode-always-different-fr