Give the "invalid directory name" dialog a cancel button

Summary: Adds a cancel button to the "Invalid Directory Name" dialog.

Test Plan:

• Try to create a new folder named . or ..
• In the resulting dialog, click Enter a Different Name -> you are prompted to choose a new name
• Enter . or .. again
• In the resulting dialog, click Cancel -> dialog disappears
• Do the above, but enter a valid name when prompted -> the folder is created using the valid name

Reviewers: VDG, Frameworks, elvisangelaccio, Dolphin, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14663