Refactor Timeline Container

Update the base item in the timeline container to be a column layout. This means that all the items can be laid out automatically without the need to set lots of manual settings and anchoring.

The overall height calculation for the delegate is vastly simplified (in fact it is removed) which deals with the fact that there were still instances where the manual calculation didn't work e.g. a delegate with a reaction followed by another message from the same user didn't give the correct bottom margin (see below)

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after:

This also improves upon the recently changed hover highlight behaviour. The previous patched moved it to cover the avatar as well as the bubble however it also covered the section and reaction when present which didn't look good. The highlight now only covers the avatar and bubble

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after:

This also cleans up some of the margins in both bubble and compact to ensure consistency.