Add KEmailValidator
That's a QValidator checking for valid email addresses. This is based on code from pim/pimcommon, but exists in similar form in multiple other copies.
KCompletion is the closest Framework we have for this I think, but it's not perfect given the needless QtWidgets dependency here. For all existing users it's an improvement nevertheless though.
@mlaurent this changes a bit of your unit test code from GPL to LGPL for consistency here, is that ok?