Qt-based media player

This patch implements a simple Qt-based media player which is actually a
graphical user interface for the SDL-based 'avplay' media player from
'libav'. It starts 'avplay' as a child and shows its graphical output in a
'QNitpickerViewWidget'. The widgets for controlling the player state send
the according keyboard and mouse input events to 'avplay'.

The 'qt_avplay' player supports the following configuration options:

<mediafile name="..."/>
-> name of the media file to play

<framebuffer_filter name="..." ram_quota="..."/> (may appear multiple times)
-> name of a framebuffer filter service to filter the video output

Fixes #222.

os/include/input/event_queue.h

@@ -14,6 +14,7 @@
 #ifndef _EVENT_QUEUE_H_
 #define _EVENT_QUEUE_H_
 
+#include <base/printf.h>
 #include <input/event.h>
 #include <os/ring_buffer.h>
 

os/src/drivers/input/ps2/ps2_keyboard.h

@@ -15,13 +15,13 @@
 #define _PS2_KEYBOARD_H_
 
 #include <base/printf.h>
+#include <input/event_queue.h>
 #include <input/keycodes.h>
 
 #include "input_driver.h"
 #include "serial_interface.h"
 #include "scan_code_set_1.h"
 #include "scan_code_set_2.h"
-#include "event_queue.h"
 
 class Ps2_keyboard : public Input_driver
 {

os/src/drivers/input/ps2/ps2_mouse.h

@@ -15,9 +15,9 @@
 #define _PS2_MOUSE_H_
 
 #include <base/printf.h>
+#include <input/event_queue.h>
 #include <input/keycodes.h>
 
-#include "event_queue.h"
 #include "input_driver.h"
 
 class Ps2_mouse : public Input_driver