fclones-gui: add custom flags field

[kapitainsky]

e.g. now using GUI it is not possible to specify hash to be used during grouping.

There are and always will be plenty of things GUI is maybe missing.

Universal solution would be to add custom flags field so I could add e.g. --hash-fn blake3

Easy to use is great idea but should not limit other users:) Otherwise you will be soon flooded with issues - I need flagX - I need flag Y etc.

custom flags - will make everybody contented at least if not happy