KoFileDialog rather belongs to kowidgetutils than kowidgets
KoFileDialog belongs to "Tier 0"
Also move filedialogtester to widgetutils:
- remove all i18n - not needed for the test
- rename to kofiledialogtester
- move to tests/ subdir
- remove main.cpp which is too general file, place main() in KoFileDialogTester.cpp
Differential Revision: https://phabricator.kde.org/D360