Updated POTFILES.in
Replace Cogl backgrounds with gradients.
timeline/elements: Use a thinner (1px) border around clips and transitions
Define some more harmonized default colors for timeline elements/clips

Also add comments for possible colors to be used as gradients eventually,
and make transitions not use rounded corners on the timeline.
check: no need to change state to check plugin

No need to change the state of the plugin to READY to confirm it
exists. Element factory make will return None when the element isn't
in the registry.
check: Check for Gnonlin availability before getting its version