This PR makes `event_timeout` and `event_timeout_ztimer` two distinct
pseudomodules, where the only api difference is in the init function.
If only `event_timeout_ztimer` is selected then no default ZTIMER
backend is selected and the old init function is not implemented.
If only `event_timeout` is selected then `xtimer` is used unless
`ztimer_usec` is included. In which case the `xtimer` wrapper on top
of `ztimer` is used and `xtimer` is not directly selected. This
allows for the legacy api to be supported with `ztimer_usec` as
a drop-in replacement.
If `event_timeout` and `event_timeut_ztimer` are selected then
`event_timeout` SRC file is excluded from compilation.
Allow using `event_loop_multi()` to handle event queues of multiple priorities
in an single thread. In the extreme case, all three event queues are handled
by a single thread (thus saving two stacks). This comes for the price of
increased worst case latency, as already running event handlers will no longer
be preempted by higher priority events.
With this, all three event queue priorities are always provided. Using modules,
the old behavior of one thread per event queue can be restored for better worst
case latency at the expense of additional thread size.
