This adjusts the positioning to work more the way we want:
* Tooltip is position on the top or bottom edge of the target depending on where
space is available
* Tooltip and chevron are centered
In addition, more bits borrowed from `ContextualMenu` are not needed, so they
have been removed for simplicity.
Part of https://github.com/vector-im/riot-web/issues/9753
Part of https://github.com/vector-im/riot-web/issues/9716
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