When the selected resource/active editor is not launchable, the user should
have the choice to launch the previous launch or launch the project containing
the selected resource. Currently, they only have the option to launch the
containing project when the selected resource is not launchable.
The net effect ist that when the user selects something that is launchable,
it will be launched. When the user selects something that is not launchable,
the previous launch is re-used. This makes contextual launch less of a
departure from the previous Eclipse release (which always launched last).
The first time this situation occurs, the user will be prompted, which
will set the corresponding launch preference (so they are not prompted
in the future). The option can still be changed via the preference dialog.