improve library/playlist clip import by detecting duplicate producers

Review Request #129034 - Created Sept. 26, 2016 and submitted

The DiveO

As it seems, MLT during export produces duplicate producers for image clips, title clips, and color clips. It is arguable if this is an MLT issue, or not. Anyway, in order to improve the situation to Kdenlive library users, this patch adds a duplicate producer detection to the playlist import.
- duplicate producers are detected only for MLT services pixbuf, kdenlivetitle, color/colour.
- duplicate producers are detected on the basis of a hash value calculated over properties kdenlive:clipname and kdenlive:foldername, as well as:
- kdenlivetitle: property kdenlive:xmldata
- pixbuf, color/colour: property resource.
This patch ensures that only a single bin clip will be used for these duplicate playlist producers.

In addition, this patch improves on user error reporting in exportTimelineSelection.

Tested on recent git master.

The DiveO
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted with commit 380e508f3e5712b75aebc22ae8dceb92efcf3a9f by Jean-Baptiste Mardelle to branch master.