Fix proxy settings implementation
This fixes several proxy setting issues:
- commit 1: In the current implementation the "system default" setting actually sets it to "NoProxy". This commit fixes this to actually use the system settings.
- commit 2: Introduces a "No Proxy" setting which can be useful to override a system-level proxy configuration.
- commit 3: Adds an implementation that also makes Qt Multimedia and Mpv obey the app-defined proxy settings. Those backends don't obey qt proxy settings, but we can make them use a proxy by manipulating the
$http(s)_proxy
environment variables.
I've split them into separate commits in case you only want to have part of the implementation.