qt: Deinterlace can be associated with codec arguments

Previously, one could not convert a video into a video with both:
 * deinterlace checked (in the Convert dialog)
 * custom options (in "Profile edition" dialog, "Video codec" tab)
This was due to incorrectly handled braces in a SoutChain.

This makes VLCProfileSelector::getTranscode() return a SoutChain instead
of a plain QString, which eases the handling of the sout chain.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>