Refactor plugin loading for WinRT

Specify the root directory to be the package root. Only plugins
inside the root can be opened (actually also only files). Furthermore
current defaults to the package root now, which in most cases is
identical to previous behavior.

When attempting to load a plugin the path can either be specified in
host format "C:/..." or as plugin absolute "/platforms/...". Check for
both, with preference of latter case, like when qt.conf is used with
/ being used as plugin path.

Change-Id: I7e3da293362488b62a3357c4882ebf5e048dcf95
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
QStandardPaths: Add GenericConfigLocation to the documentation

The actual feature comes from a merge from the stable branch.

Change-Id: Id067fb2b26cb3c6b78c7705b77bd741e3bd8f090
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
use apartment threading for CoInitializeEx

otherwise we are risking a lock by freeing resources from other
threads when calling CoUnitialized()

Change-Id: I1fe09d7d3b1674a00b44ababfc90da79580278b3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>