Currently we check whether the destination URL exists only if we are
pasting text (or if the mimedata has only one format).
If we are pasting e.g. images copied from Spectacle, we unconditionally
overwrite the destination URL.
This patch moves the StatJob+RenameDialog from getNewFileName() to a
new getDestinationUrl() function, which we can also call from chooseFormatAndUrl().
BUG: 394318
FIXED-IN: 5.47