0.7 beta: Reproducable timeline corruption / crashes

While I was editing a medium sized project on kdenlive (about 300 cuts, many of which had transitions) I had to delete large portions of the time line to work around the lack of a spacer tool in the current release. I noticed several deal-breaking issues that deserve further analysis:

- when selecting a large amount of items (using shift and drawing a square on the time line), kdenlive crashes or fails to move them when trying to move them. It often crashes when deleting a large number of items.

- When items are deleted from the time line, especially longer items, the time line sometimes becomes desynchronized -- videos are shown in incorrect areas on the time line. That is, I may believe a video ends or has not yet begin because that's how the time line shows it, but the video will actually show up in the project monitor.

- Under the same conditions as above (large or many items deleted from the time line), the time line becomes desynchronized -- videos and transitions at other sections of the time line have moved themselves. It is possible that only the videos moved themselves and the transitions remained unmoved. However, this makes working with large projects nearly impossible as the time line cannot be trusted.