Update translations from Transifex:

 - Arabic
 - Basque
 - Bengali
 - British English
 - Chinese (Simplified)
 - Croatian
 - 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.
Capture page: Cosmetics.

Reduce the visibility of some member variables and functions.
Option pages: Show a tooltip when the text of a combobox does not fit.
Add a tooltip to describe the two "fast seek" methods.

Fixes #4177.
Formats option page: Reworked UI.

Replace the text buttons by icon buttons and add a new button to clear all associations.

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...