tst_qicon: Resolve all files using QFINDTESTDATA().

The local files in availableSizes() were not resolved.
Introduce member variables for the file names, resolve them
in the constructor and add initTestCase() to verify.

Task-number: QTBUG-39287
Change-Id: If841e904700fe76b6c9265124ccba7764911fdc9
Reviewed-by: aavit <eirik.aavitsland@digia.com>
QIcon: Prefer high-quality images of Windows .ico files.

Refactor the code QPixmapIconEngine::addFile() using a convenience
class for reading all images.
Special-case .ico-files: Read images into a list and replace by
higher-quality ones.

Task-number: QTBUG-39287
Change-Id: I32ab6c77a276dc5d4d9a8f7b216c81149b8772b8
Reviewed-by: aavit <eirik.aavitsland@digia.com>
CMake: Load plugin files unconditionally if present.

Task-number: QTBUG-39171
Task-number: QTBUG-39451
Change-Id: Ie66bd6b787a0957fd6f7ea673b158ab5de3cc38f
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>