Don't keep temp CReadState on CurlFile::Seek

Apparently there are a lot of servers around that limit to two
concurrent connections.  This also eliminates a potential leak of the
temporary CReadState should the early return be taken.
Try CurlFile::Seek without multisession on failure
Minor refactor of CurlFile::Seek
fix crash when changing language from non-GUI thread (fixes #15113)
fix crash when changing the skin zoom from non-GUI thread (fixes #15114)
[omxplayer] Request to be notified about framerate changes