Allow customizing merged output file name

[mifi]

Currently it will get a unique name: Movie-cut-merged-000012245.mp4 in order to not overwrite existing files. Maybe we can do this:

• see if original file name doesn't exist, e.g. if output directory is different from input directory
• if it doesn't exist, use that name instead

Or we can show an input field in the export dialog where the user can input the merged output file name if "merge" mode is enabled.

See also #916 #96