subtitle: player level logic is common
qml: support ass subtitle
uninstall filters in target dtor and ~Filter
qml ass subtitle item update less if no image

TODO:
update only if image changed
better way to get ffmpeg supported subtitle extensions
subtitle: search dirs support
subtitle: add signal loaded(path)
qml subtitle image fillMode
update plugins.qmltypes
added support for avdevice by forcing input format
Merge pull request #306 from sherpya/avdevice

added support for avdevice by forcing input format
reset AVInputFormat on close
qml: correct item geometry. issue 304
fix strstr crash
initial value = 0 in AVClock.reset()

test: play v4l2 camera, then a normal file
Merge remote-tracking branch 'upstream/master'