This refactors the internals of the `Timer`, such that it's impossible
for different fields to contain conflicting information. This is done by
factoring out an `ActiveAttempt` struct that only exists when there is
an active attempt. Additionally inside it differentiates between the
attempt having ended or not. The not ended state has a split index and
an optional pause time, whereas the ended state has the date time of
when the attempt ended.

This does not yet touch the public API of the `Timer`. Factoring out the
notion of an active attempt and making sure invalid states are
unrepresentable is done so we can soon explore the possibility of
storing an active attempt in the splits file.