Style-specific file selectors fail if the first letter isn't uppercased

For example, create the following directory structure:

Blah.qml
+material/Blah.qml

Blah.qml will always be chosen when -style material is passed as an application argument, unless +material is renamed to +Material.

This goes against what is documented in http://doc-snapshots.qt.io/qt5-5.7/qtquickcontrols2-fileselectors.html.