Add QFileSelector API

For easy cross-platform and cross-device UIs, automatic asset swapping
based on filename is being developed. This API provides the logic for
the swapping, so that applications can use it themselves with the same
logic as any automatic swapping done in application templates.

Selector set is initially minimal, aiming for just platform selection
and enabling a common selection mechanism for Qt platforms to use.

Change-Id: I219517d740fa7385e923a9e09cb7e241378f857a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Un-export qt_registerFont() internal function

QPlatformFontDatabase::registerFont() method must be used instead.

Change-Id: I80aa5567a748a980f689c90125f8bcc20c304ee2
Reviewed-by: Lars Knoll <lars.knoll@digia.com>