KNewFileMenu: Forbid saving file or folder with ~USERNAME as name

Akseli Lahtinenrequested to merge
work/akselmo/kfilewidget-fix-tilde-expansion into master

When creating new file or directory, check that any name which starts with ~ does not resolve into a home directory on the machine.

This is due to an UNIX convention where "~user" can resolve into "/home/user".

On Windows we do not have to worry about resolving this file.

Added an autotest for this as well.

CCBUG: 317513

Edited by Akseli Lahtinen

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