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.

platform/x11/os_x11.cpp

@@ -1941,6 +1941,39 @@ bool OS_X11::_check_internal_feature_support(const String &p_feature) {
 	return p_feature == "pc" || p_feature == "s3tc";
 }
 
+String OS_X11::get_config_path() const {
+
+	if (has_environment("XDG_CONFIG_HOME")) {
+		return get_environment("XDG_CONFIG_HOME");
+	} else if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".config");
+	} else {
+		return ".";
+	}
+}
+
+String OS_X11::get_data_path() const {
+
+	if (has_environment("XDG_DATA_HOME")) {
+		return get_environment("XDG_DATA_HOME");
+	} else if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".local/share");
+	} else {
+		return get_config_path();
+	}
+}
+
+String OS_X11::get_cache_path() const {
+
+	if (has_environment("XDG_CACHE_HOME")) {
+		return get_environment("XDG_CACHE_HOME");
+	} else if (has_environment("HOME")) {
+		return get_environment("HOME").plus_file(".cache");
+	} else {
+		return get_config_path();
+	}
+}
+
 String OS_X11::get_system_dir(SystemDir p_dir) const {
 
 	String xdgparam;