Add commands to seek video to audio selection bounds

[dvdkon]

This is a simple PR that adds two commands that take the current region selected in the audio panel and seek video playback to either the start or end of the region.

I'm new to Aegisub both as a user and as a developer, so I'm open to suggestions. Also, I have to say that this part of the codebase was a joy to work with :)

[arch1t3cht]

Thanks for this PR, too. But here, too, I have to ask what exact use case you are addressing with this:

There are already commands video/jump/start and video/jump/end to jump to the actual start and end times of lines, so the added feature here would be being able to jump to the not yet committed start/end times.

In case your intention here is to use the audio selection simply as a way to "input" a timestamp using the audio display (rather than something to do with actual planned line start/end times): You may not be aware that you can already middle-click the audio display to jump the video to the corresponding point in time. (This is something that many users aren't aware of - I don't think it's described in the manual and I myself only discovered it when reading the code - but it's one of Aegisub's best features in my opinion and helps my workflow immensely.)

If your use case for this is something that's not solved by being able to seek using middle-click, it'd be nice to know what it is exactly. Once again I'm not opposed to adding this in principle, but I'd like to make sure it's actually necessary first.

(If this is added, the commands would also need to validate that audio is loaded.)