[launch frontend] Support tag namespaces

[ivanpauno]

Feature request

Feature description

Actions and substitutions can be exposed from several packages.
Name collisions are possible, and sometimes desired (e.g.: GroupAction in launch_ros with extra ns argument, see ros2/design#207 (review)).
Adding namespace is a way of solving the problem.

<group>
  ...
</group>

<ros::group ns="my_ns">
  ...
</ros::group>

Using tags would be also desired:

<using name="ros::group"/>
<using namespace="ros"/>

Using tags should be allowed in launch and group tag (and any other future tag that accepts a list of subentities).

Implementation considerations

Actions and Substitutions are exposed to launch.frontend using decorators.
The decorators populates two dictionaries, action_parse_methods and substitution_parse_methods, in the format:

{'TAG_NAME': <action/substitution>, ...}

It can be easily change to a nested dict:

{'pkg_identifier': {'TAG_NAME': <action/substitution>, ...}}

This, together with a name and namespace context should solve the problem.
The parsing functions of LaunchDescription and Group should check/push/pop this context.
We could have a parse_children methods in Parser class, for better code reuse.

Alternatives

Some sort of overriding is also possible. It doesn't solve some corner cases, e.g.:

• Launch exposes group action.
• Two different packages inherit from it and use the same tag name.

[hidmic]

@ivanpauno I'd be inclined to use a single colon : instead of two :: to be consistent with XML namespaces syntax. Also, I'd make the operator markup-language dependent e.g. using colons : on a YAML launch file will confuse the parser, better use dots ..