Add links to Streamlink website and download page to Streamlink settings page
Make streamlink custom path usage more explicit
Change how streamlink runs, it now works perfectly on Linux so that if you don't have a custom path set, it will try to just run "streamlink" in your PATH variable
This needs testing on Windows again
Things that were once singletons are no longer singletons, but are
instead stored in the "Application" singleton
Some singletons still remain, and some renaming/renamespacing is left
