Move dropdown controls for video and audio codec to the respective boxes

[andimachovec]

Moving the codec selection below the enable/disable checkboxes for video and audio makes it much easier to disable video or audio streams for encoding. @humdingerb: What do you think?

[humdingerb]

I like it. Maybe move the codec pop-ups into the grid layouts below them and use "CreateLabelLayoutItem()" etc for them. That'll align the labels and widgets nicely.

Also, I think we can do away with the label "Output file format:" and move the format pop-up up, between the output file text control and the play button.

[andimachovec]

I moved the codec pop-ups to the respective grid layouts, looks much better. Didn't have much luck with the output file format pop-up. No problem moving it to the grid layout technically but I just didn't like how it looked. Either the play buttons are not aligned or the one next to the input file textcontrol is too far away (if I put the play buttons in the same grid column).

[humdingerb]

I think I managed to move the format popup up. I'll merge this PR and create a new one for that for your inspection.