Summary:
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

Test Plan:
- Paste some image into an existing file and check the rename dialog.
- Check that pasting text still shows the rename dialog.

Reviewers: dfaure

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13145