Resolves #5305

Breaking change... Opening this as a draft for now, since I'm aiming for the next major version release when py37 support gets dropped in two months.

Please read #5305 for the full details about the motivation of these changes.

Changes

1. Rewrite find_default_player()
2. Turn --player into a path-only argument and remove support for specifying a whole player command-line with additional arguments. Player arguments now must be set via the --player-args argument.
   As a result, subprocess.Popen() and subprocess.call() now receive an argv with a properly defined and safe player path with additional player arguments. The player detection also doesn't rely on a weird path basename workaround anymore.
3. Remove the old --player-args {filename} fallback variable. Only {playerinput} is now supported.
4. Refactor the PlayerOutput's player detection and the player arguments builder for better code quality and easier extensibility.
5. Actually check/verify the player path via shutil.which(), so a better error can be raised when the path is incorrect, or when the player path is not executable, or when the single executable name can't be resolved from the system's PATH env var. Additionally raise a warning with more info when the player path begins with a single or double quotation mark, as this is now unsupported. Detecting player args in the --player value is not possible, so a warning can't be raised for those cases.
6. Add {playertitleargs} to --player-args, so the order of automatically generated player titles can be customized. By default, player title args come in front of the player args set by the user, so user data can override (if the player supports overriding previous args).

More details in the commit message bodies...

Notes

• Since custom player arguments now can only be set via --player-args, this makes the addition of player title arguments more strict. Previously it was possible to set arguments in front of the automated player title arguments, but this is not possible anymore because the order is always path -> title-args -> custom-args, where custom-args has support for the {playerinput} variable, so the input argument can be set freely here. I was thinking about adding another variable ({playertitle}) that replaces the title-args segment and that automatically gets added to the --player-args if it's missing, similar to {playerinput}. Then the player title args can also be set freely, like the player input arg. Thoughts?
• The updated default player lookup doesn't have tests. I've tested it manually though...
• No changes to the PlayerOutput implementation itself (which I'm still unhappy about)
• The Streamlink config files will all be outdated with these changes. This is especially bad on Windows where the installer only writes the config file if it doesn't exist yet. This will require a special note, or maybe even a new option in the installer which lets it overwrite it with the default config.

Please give this branch a thorough test. It would be pretty annoying if something was missed in these changes.