Update translations from Transifex:

 - British English
 - Chinese (simplified)
 - Croatian
 - Czech
 - French
 - German
 - Greek
 - Japanese
 - Hungarian
 - Malay
 - Polish
 - Portuguese (Brazil)
 - Slovak
 - Spanish
 - Swedish
 - Turkish
 - Ukrainian
 - Vietnamese
Add an Arabic translation.

The composition of the translation team can be found on Transifex: https://www.transifex.com/projects/p/mpc-hc/language/ar/members/.
Comment out some unused variable assignments.

Those can be useful if the functions are extended one day.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
Why "C:\tmp.srt"?
WebUI: Inform the web interface that no audio stream is available.
WebUI: Make use of new [hasaudio] info.
What the hell was intended here?!

Trying to make some sense of that code. Without much luck so far...