Clip export settings improvements

[seafraf]

• "Save clip as" field should have a default value
• Specify codecs in output formats so that the a/v codec selection can show only compatible formats
• Two pass encoding for audio? requires investigating, if it is not supported then the option should be removed
• If "Change video FPS" is not checked, the field should show the source FPS
• If "Change video resolution" is not checked, the field should show the source resolution
• Force merging when the output format does not support more than one audio stream
• Audio only exporting, see Audio support #29
• Export button leaves a permanent shadow after clicking and going back
• FFmpeg output window shows too much text