Allow filtering in the sequence diagram

[rmarx]

Right now, if you load a long trace (say 100MB+) and you're for example only interested in changes in flow control, it can be challenging to impossible to find them.

A minimal version of this feature would allow filtering on 1 or more event + frame types. A more complex version would allow a more advanced grammar to for example select fields with a particular value within an event and complex combinations ( (event type x OR y) AND (time > 5000) etc.)

It would also be nice to (optionally) show the events right before and after the filtered events to get some context.

Technically implementing the filtering itself won't be difficult (we already filter on time-ranges atm to reduce rendering overhead for example). It's mainly making a user-friendly interface for something advanced that would be the problem here.