WebClientSocket: Fixed case where return value of AsyncReader::Length() was not checked.

As reported by Coverity in CID 1084352.
Change PPageFileInfoDetails for consistency.
Audio switcher settings: Fix the spinner not being correctly disabled.

This commit fixes #3428.
Properties dialog: Disable the "Explore to" feature when the file isn't local.

This commit fixes #3440.
Playlist menu: Disable entries that don't make sense for non-local files.
Updated German translation.