server: add wit_display_create_and_run()

This function is abbreviation for
    wit_display_create
    wit_display_create_client
    wit_display_run
client: flush before kicking display

Send pending data to display before sending a request
events: don't allocate array data when size is 0

When recieving event, we allocate the data by hand. Don't allocate the
data when its size is 0
events: remove useless dbg messages
server: flush queue before processing request
client: use dispatch instead of roundtrip

it's faster
client: dont call roundtrip when it's not needed
server: remove useless dbg message
tests: events-test updated

new tests, some old updated
tests: wit-test, new test

test wit_display_create_and_run function