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.
This patch makes use of the recently added support for const RPC
functions by turning 'Framebuffer::Session::mode()' and
'Input::Session::is_pending()' into const functions.
