This patch fixes five bugs:

1) Recover warning dialog if the user tries to render animation
   frames into the directory with already existing frames. (QDir
   uses wildcard symbols instead of c-like patterns used by ffmpeg)

2) Warn the user if rendering of frame files didn't complete
   successfully. Previously, Krita just tried to run ffmped on the
   non-existing files.

3) Create the exporting directory if it doesn't exist.

4) Create the rendering directory if it doesn't exist.

5) If the user activates video-only mode and selects absolute file
   path, then export the frames into the same directory as the rendered
   file itself.