Add support for crop

[jlfwong]

In time ordered view, it would be useful to be able to crop along the time axis to the current viewport. This would allow for more meaningful statistics around where the time is going within some specific time span (e.g. a single request or a single frame).

[sandyarmstrong]

Absolutely, would love this! Sometimes I have a trace with some big noisy event that has a lot of frames that are also around beforehand. I'd like to only look at the trace from before the big noisy event so I can better analyze those frames in context.

[jlfwong]

#366 contains a version of this already implemented. I'll take a closer look at it before next release -- I think it doesn't handle multithreaded profiles well yet, and since this is a destructive operation, it would be nice to provide the ability to undo this operation without reloading the whole profile. This would be the first need for undo in the system, since this is the first destructive operation.