playbin2: handle GST_PLAY_FLAG_FORCE_SW_DECODERS

playbin2: handle GST_PLAY_FLAG_FORCE_SW_DECODERS

Added `force-sw-decoders` boolean property in decodebin2 and
uridecodebin. By default the property is %FALSE and it bypass the new
code. Otherwise the factory list is filtered removing decoders
within 'Hardware' class.

uridecodebin sets the `force-sw-decoders` property in its internal
decodebin, and also filters out Hardware class in the
autoplug-factories default signal handler.

playbin2 adds by default GST_PLAY_FLAG_FORCE_SW_DECODERS it its flags
property, and depending on it playbin2 sets the `force-sw-decoders`
property on its internal uridecodebin, also filters out the Hardware
class decoding decoders at the autoplug-factories signal handler.