tools:validate: Concider seek in PAUSED position being not exact as WARNING

In some corner cases in GNL it is totally correct that a position in a
seek in paused is not perfectly exact
timeline: handle async start
xml-formatter: message-forward is not something that should be parsed.
ges: Use the new GNL element message feature to notify async operations
ges:validate: Actually no need for overrides these days
ges:validate: Port to the new handles-states API