qtwebkit: add dependency on gstreamer and libxslt

* it's autodetected from sysroot, make it deterministic
* maybe I should allow to change gst-0.10 to 1.0 without
  rewriting DEPENDS

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
qtmultimedia: add gst-plugins-base dependency and openal PACKAGECONFIG

* this will need small patch in qtmultimedia.pro to run
  qtCompileTest(openal) only with OE_OPENAL_ENABLED

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>