Update translations from Transifex:

 - Arabic
 - Basque
 - Bengali
 - Chinese (Simplified)
 - French
 - Galician
 - German
 - Greek
 - Hungarian
 - Japanese
 - Malay
 - Polish
 - Romanian
 - Slovak
 - Slovenian
 - Spanish
 - Thai
 - Turkish
 - Vietnamese
Installer: Add a Bengali translation.
Improve the command line help dialog.

Fixes #1663.
Internal filters page: Improve the context menu handling.

- The context menu can now be triggered using the keyboard.
- The context menu won't be quickly displayed before being replaced by the default context menu when right-clicking on the scroll bars.
Formats option page: Add a context menu to replace the buttons.

The buttons were messy, hard to translate because of their size and there was no room to add a button to deassociate all formats.

Also replace the "with icons" checkbox text with something clearer now that there is more space.

Fixes #4687.
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...