This allows things like wlr-randr to work.
wlr-randr or similar can send in two new events: `output_manager_apply'
and `output_manager_test'. In output.c, their handler both call an new
`output_manager_configure' function which loops through the list of
outputs twice. The first loop applies all the requested configuration
ad checks that its not all messed up. The second loop either commits
that configuration or reverts it depending on whether it worked and
whether we're responding to a test event.
There's also now an output_manager_update function, called whenever the
output layout is changed, which copies changes from the
wlr_output_layout to the wlr_output_manager.
