Add initial support for the XDG Base Directory spec

Spec version 0.7 from https://standards.freedesktop.org/basedir-spec/basedir-spec-0.7.html
(latest as of this commit).

Three virtual methods are added to OS for the various XDG paths we will use:
- OS::get_data_path gives XDG_DATA_HOME, or if missing:
  ~/.local/share on X11, ~/Library/Application Support/ on macOS and %APPDATA% on Windows
- OS::get_config_path gives XDG_CONFIG_HOME, or if missing:
  ~/.config on X11, ~/Library/Application Support/ on macOS and %APPDATA% on Windows
- OS::get_cache_path gives XDG_CACHE_HOME, or if missing:
  ~/.cache on X11, ~/Library/Caches on macOS and %APPDATA% on Windows

So for Windows there are no changes, for Linux we follow the full split spec
and for macOS stuff will move from ~/.godot to ~/Library/Application Support/Godot.

Support for system-wide installation of templates on Unix was removed for now,
as it's a bit hackish and I don't think anyone uses it.

user:// will still be OS::get_data_path() + "/godot/app_userdata/$name" by
default, but when using the application/config/use_shared_user_dir option
it will now use XDG_DATA_HOME/$name, e.g. ~/.local/share/MyGame.

For now everything still goes in EditorSettings::get_settings_dir(), but
this will be changed in a later commit to make use of the new splitting
where relevant.

Part of #3513.

drivers/unix/os_unix.h

@@ -62,8 +62,6 @@ protected:
 
 	String stdin_buf;
 
-	String get_global_settings_path() const;
-
 public:
 	virtual void alert(const String &p_alert, const String &p_title = "ALERT!");
 	virtual String get_stdin_string(bool p_block);
@@ -108,11 +106,8 @@ public:
 
 	virtual void debug_break();
 
-	virtual String get_installed_templates_path() const;
 	virtual String get_executable_path() const;
 	virtual String get_user_data_dir() const;
-
-	//virtual void run( MainLoop * p_main_loop );
 };
 
 class UnixTerminalLogger : public StdLogger {