[LMP] Don't pull sourceobject.h in player.h.

Otherwise changes in SourceObject lead to lots of recompilation :(
[LMP] Use LastCurrentTime() in GetRemainingTime() as well.
[LMP] Ignore NEW_CLOCK and ASYNC_DONE messages.
[LMP] Handle Buffering messages.
[LMP] Handle source object changes & set proper UA string.
[LMP] Temporary soup plugin rise handling.

After first complex page load WebKit instantiates its own WebKitWebSrc
which handles all http(s) stuff and can't be used outside WK, thus
killing streams playback. This fix temporary rises the standard
souphttpsrc priority before setting "uri" property and reverts it back
after source has been constructed. This way souphttpsrc is used in LMP
and WK happily continues with WebKitWebSrc.