[python] adds a background progress dialog
[release] version bump to 13.0 alpha3 as we start new merge window.
[jenkins] - bump ios/atv2 builds to default to ios sdk 4.3 which comes with xcode 3.2.6 which is the minimum required version as of our own readme
Merge pull request #2510 from MartijnKaijser/version_april

[release] version bump to 13.0 alpha3 as we start new merge window.
Merge pull request #2484 from nuka1195/DialogProgressBG

[python] adds a background progress dialog
Movie set management: add to set, remove from set, new set creation - warnings
Movie set management added: allow add/remove movies within the sets view