Move files instead of copying and then deleting the source (when dragging)

[FichteFoll]

When dragging files to another folder (in the sidebar, maybe in other places as well), qimgv copies the files and then removes the originals. It also manually restores the original timestamps (see #274), but the process is slow and especially for large video files in the gigabyte dimension this takes quite some time and locks the UI during the process. It would instead be much more performant if the files were moved instead.

Related: #274

[FichteFoll]

~~Also, slightly related, I noticed that the default action of dragging is to move even though it shows the icon with a + (which usually means copying). I can also open a separate issue for that.~~ Split into separate issue.

[nick-s-b]

Just hold Shift when dragging and it will move. This is actually consistent with KDE and GNOME ecosystem and UX.

[FichteFoll]

For me, the file is moved regardless of whether I hold shift or not. Or, well, it is copied and then the original is removed, as easily observed when draggin video files of a few megs, which causes qimgv to freeze until the operation is done.