PlayerNavigationDialog: Remove unreachable code.

Remove the assert since it can fail in normal situations.
PlayerNavigationDialog: Initialize m_bChannelInfoAvailable.
WebServer.cpp: Add some error checking for debug builds.
Cosmetic: Tweak the OSD message for audio delay changes.

Make it consistent with the OSD message for subtitle delay changes.

Fixes #4989.
Grammatical fix: Units should be preceded by a space in English.

Translations completely identical to the English version have been reset to encourage the translators to double-check that the translation was really correct.
Fix: The "Channels" sub-menu was not translated anymore.
DVB: Add a special case for SetChannel at first start or after clearing all channels.

We don't want to assert in this case since it's expected that we have no channel to set.

Also make nDVBLastChannel a signed integer for consistency with the rest of our API.
DVB: Make sure the preferred channel number always stay consistent.

Moving a channel up/down or sorting by LCN wouldn't update the preferred channel numbers which caused strange behaviors when going to next/previous channel or when using "Navigate" menu.
Don't force any background color for the Shockwave graph.

This commit fixes #1747.
WebUI: Inform the web interface that no audio stream is available.