Backport "seek to the first clip edge before or after the playhead" functionality into 1.0 development branch

Hi,

I've just tried the master development flatpak version of Pitivi and I was delighted to find that very practical functionnality : seek to the first clip edge before or after the playhead.

Could you consider backporting this into 1.0 development branch ?

Pros : long-awaited functionnality, rather basic (=vital) for a video editor, that can be find in current versions of Kdenlive & OpenShot for instance Cons : adding instability in 1.0 development branch (have no idea about that : I let you judge)

Thanks

(BTW, I didn't find – neither in stable nor in master development – the way to add markers to the timeline, to allow user to quickly navigate through the timeline. Tell me if the functionality – that can be find in current versions of Kdenlive & OpenShot for instance – is not already supported in which case I will create a bug for it. Thanks!)