Add the ability to choose destination folder in the transcoder dialog.
Adjust the ui and clean up code.

The selection of a destination folder is now done by pressing the button 'Select...'.
The last selected folders along with the option 'Alongside the originals' are
available in the box 'destination'.

The methods TranscodeDialog::TrimPath and TranscodeDialog::GetOutputFileName are now
const.

Minor formatting fixes.
Merge pull request #4105 from paperbagcorner/transcoderdestination

Add the ability to choose destination folder in the transcoder dialog.  Fixes #2776.