AK+Everywhere: Change URL::path() to serialize_path()

This now defaults to serializing the path with percent decoded segments
(which is what all callers expect), but has an option not to. This fixes
`file://` URLs with spaces in their paths.

The name has been changed to serialize_path() path to make it more clear
that this method will generate a new string each call (except for the
cannot_be_a_base_url() case). A few callers have then been updated to
avoid repeatedly calling this function.

Userland/Applications/SoundPlayer/SoundPlayerWidgetAdvancedView.cpp

@@ -161,7 +161,7 @@ void SoundPlayerWidgetAdvancedView::drop_event(GUI::DropEvent& event)
             return;
         window()->move_to_front();
         // FIXME: Add all paths from drop event to the playlist
-        play_file_path(urls.first().path());
+        play_file_path(urls.first().serialize_path());
     }
 }