Make `DelegateSizeHelper` take a `QQuickItem` rather than a width value and move `TimelineDelegate` to use it rather than duplicting the code.
This requires the creation of `LibNeoChat` so that both the main and timeline modules can get access to the class.
Note: ideally more stuff will go into `LibNeoChat` but it turns out our dependencies are kinda spaghetti like and so will take some untangling.