Fixed several bugs in the loader implementation and seek command

With this commit several of the indefinite loader bugs should be fixed.
We've done this by proper use of deferreds in
LYT.player.seekSegmentOffset so that the loader will disappear on both
success and failure

We've also implemented a chain/queue of seek commands, so that several subsequent clicks on Forward15/Backwards15 will be chained.

One of the most severe bugs we fixed was that several `load` and `seek` commands could be active at the same time, though (of course) still interacting with the same `<audio>` element. This led to several race-conditions and would in a lot of cases result in the hanging loader.

(*we* are @LDHGithub and I)