Move XMML and URL escape functions to common/utils

So we don't need to include notifications.h on the Wine side. The
alternative would be to put ifdef guards around everything involving
notification sending in `notifications.cpp` but that would be even more
problematic.

src/common/notifications.h

@@ -45,14 +45,3 @@
 bool send_notification(const std::string& title,
                        const std::string body,
                        std::optional<ghc::filesystem::path> origin);
-
-/**
- * Escape XML entities within a string. Used inside of desktop notifications.
- */
-std::string xml_escape(std::string string);
-
-/**
- * URL encode a file path. We won't escape forward slashes, and `path` should
- * not yet include the `file://` prefix.
- */
-std::string url_encode_path(std::string path);