Possibility to buffer actions for certain dynamic pad that is yet to be linked

[Qizot]

It is a common usecase that a certain element may receive a single input e.g. FLV demuxer and can have several dynamic outputs depending on the content (audio/video streams present, a single one or both).

Right now the flow of making use of those dynamic pads looks like this:

• demuxer sends a parent notification that the dynamic pad needs to be linked
• demuxer caches all the actions produced by the input (assuming push mode)
• once the pad gets added replay all the buffered actions

and of course if the pad doesn't get linked in certain timeout value or certain amount of actions then the element throws.

It would be nice to have a mechanism where we could define some dynamic pad buffering and replay those actions right after the pad gets connected.