GStreamer: cleanup system includes.

Replaced some linux includes with <private/qcore_unix_p.h> to be more
portable.
Use qt_safe_open() and qt_safe_close() instead of ::open() and
::close().

Task-number: QTBUG-38061
Change-Id: Idb3f50d070ec1701cafba98e79e2e85824653a3a
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
DirectShow: fix media seeking while stopped.

When changing the position of a media player while stopped, playback
was automatically resuming. We now cache the value and actually set the
position only when starting playback.

Task-number: QTBUG-38068
Change-Id: Ib35302e01967d70350125f9ad920551df2a3c25b
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
make MODULE_PLUGIN_TYPES assignment actually have an effect

it must be done before load()ing qt_plugin.prf.

Change-Id: I0ad31f8d9761b2d059d656c9e4f674c8a7cc88a6
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
let videonode plugins declare that they extend QtQuick

Change-Id: I4facc9ca0616927f7b1101cecb989fece38c2f7a
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>
Tell glib not to complain about the use of API recently deprecated

When building with glib 2.32 or later, without this #define, it prints a
warning saying:

function "g_value_get_char" was declared deprecated ("Use 'g_value_get_schar' instead")

Since g_value_get_schar is new in 2.32, we can't use it without #ifdef
around the call point. Since the old and new functions are identical in
behavior, just ask glib not to complain about the use of old functions.

I don't know which is the minimum version of glib we require, so I went
for the lowest available macro.

Change-Id: I33f9aa8497fc6bd50fffb3c59ee7e8e7fcdfe110
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>