rtt: do not invoke user callbacks or trigger the component if the component is not running

This commit fixes issue https://github.com/orocos-toolchain/rtt/issues/61.
Additionally, a new dataOnPortHook() is introduced that can be overwritten by the user to influence
whether the component is triggered or not.

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
rtt: moved isRunning() check from TaskContext::dataOnPort() to default TaskContext::dataOnPortHook()

This allows the user to decide in an overwritten dataOnPortHook() implementation whether the user callbacks
should be enqueued or not, indepedent of whether the component is running or not. Note that the callbacks are
not executed until the component is started unless #70 is merged.

See https://github.com/orocos-toolchain/rtt/commit/d03c66b69912714b851818d471beb3282f19bad3#commitcomment-8247917.

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge remote-tracking branch 'origin/fix-61'