Warn user before renaming the file/folder to start with a ' . '

Summary: For normal "casual" linux users, renaming the file/folder starting with dot may get irritating, they will be wondering their file is deleted.

Test Plan:
1. Make new file/folder.
2. Rename it to .foo
3. Question dialog appears.

Reviewers: elvisangelaccio, ngraham, pino

Reviewed By: elvisangelaccio, ngraham

Subscribers: cfeck, emateli, elvisangelaccio, pino, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15980