Revert to default binding mode before reloading the config.

Revert to default binding mode before reloading the config.

If a user reloads the config while in some binding mode, the binding mode
will revert to the default, but no event will ever be fired, causing a
broken i3bar mode display.

This patch explicitly reverts to the default binding mode before reloading
the config. We reload rather than switch to the binding mode after having
reloaded the config because there's no guarantee that mode will even still
exist.

fixes #2228