trim empty frames from beginning and/or end

[Christopher Lovejoy]

currently it's easy to press and hold the right or left arrow keys and add hundreds of empty frames to the end or beginning of the film. There should be an equally easy way to remove them: a menu option to trim animation;

This should be nested under some grouping in the menu, as it shouldn't be needed often.

[Christopher Lovejoy]

Adding frames is no longer a repeating function. The cause for the request isn't relevant for now.