chore(linux): call `reset_context` instead of `...focus_in`

Both `ibus_keyman_engine_reset` and `ibus_keyman_engine_focus_in`
are methods that get called by ibus. Having `ibus_keyman_engine_reset`
call `ibus_keyman_engine_focus_in` makes it much harder to see in
log files which methods get really called by ibus.

This change replaces the call of `...focus_in` with `reset_context`,
which is what `...focus_in` does as well. The only other thing that
`ibus_keyman_engine_focus_in` does in addition to that is calling
`ibus_engine_register_properties` - which shouldn't be necessary on
a reset (besides that we don't seem to do anything with the one
status property we register).