This seems to be an unintentional change in compact mode caused by 054f87cae2,
where an if check for "straight line" or "fill width" mode ended up
being removed. But this was needed for availableWidth() to return the
correct width for compact mode, otherwise it got weirdly centered and
ended up with a limited width.
Rework MessageDelegate to be mostly a cpp class. This allows us to only load the components that are actually needed saving memory.
In testing using memtest it saved ~30% versus the current implementation.
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.