more Kdenlive library love: improve playlist expansion

Review Request #129023 - Created Sept. 26, 2016 and submitted

The DiveO

This patch improves the existing expansion of playlist clips in the timeline as follows:
- improves on error reporting when there is not enough free track space to expand the clip, more precisly spelling out what amount of free track space is needed, as opposed to free tracks.
- improves overall error reporting in the context of expanding playlist clips by adding user-visible reporting where we only had debug messages so far.
- improves on collision detection, improving workflow for some Kdenlive library users, see below.
- playlist clip expansion is no down instead of up: this better matches the overall timeline concept, where in the timeline transitions belong to the "b track" upper track, as opposed to the "a track". Transitions extend below the track they are placed on.
- if there isn't enough free room on the timeline below, then, as before, the playlist clip is attempted to be shifted up for expansion in order to find enough free track space.
- following the timeline model, expand clip does not regard a transition immediately above the playlist to be a collision. In fact, as playlist clips cannot possess transitions with a "b track" above their topmost playlist track, there can't be such a collision anyway. In turn, this patch improves the workflow for those users that have things like overlay logos on a top track and need to expand a playlist clip immediately below it.

On recent Kdenlive git master.

The DiveO
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted with commit e6561652db31de58ea4570da0edb4af3e7a02379 by Jean-Baptiste Mardelle to branch master.