Fix rendering of animation frames if the directory not exist

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.