rename "metadata::Component" to "metadata::TaskContext"

mind while merging: has dependency to an apropriate change in
"base/orogen/std", where this struct is pulled into the typekit...

fixes the following warning:
    Warning: Identifier 'Component' clashes with CORBA 3 keyword 'component'

for a discussion see https://github.com/rock-core/tools-orogen_metadata/pull/2

see also https://github.com/rock-core/base-orogen-std/pull/3

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>