Adding sub titles?

I have a video and a .srt sub title file. I've gotten Handbrake to combine these two files into a new MP4 video that has a text stream inside and now I can turn the sub titles on and off when I watch the video. Works well except:

1) Handbrake re-encodes the video and the process is slow but worse, it changes the size of the video by quite a lot in either direction depending on how I set the compression. I want it to NOT RE-ENCODE. All I want is the keep the video UNCHANGED and multiplex the text into the file. Is there some app, better than Handbrake for adding the text stream? Or maybe Handbrake has a setting where it can be told to use the video as-is with no receding?

2) I want to add subtitles to my own videos. Soft titles that can be turned off and on. I'm using FCPX. Can I edit a text stream in FCPX?

3) is there a way to add a text stream that has formatting information so I can specify things like italics