Warn user when move/copy potentially involves more data than destination has space for

[OmegaPhil]

Forked from #9

#9 (comment):

Responding to the free space note, spacefm does 'know' this (or perhaps an approximation) as I use this figure at the bottom-left status bar when judging larger transfers etc. It would be useful say to warn the user prior to commiting the move/copy etc (another point against denying the move/copy outright would be that the user could clear some space up as the copy/move operation works).

#9 (comment):

It couldn't be the default (on compressed filesystems, for example, you don't know how much space the file will take until it's written), but perhaps it could be made an option at some point - it would be useful in many cases, I agree. But that is unrelated as far as this bug is concerned - in no event should the original be removed until the destination is complete.