[plug-in] output channel visibility

[benoitf]

If inside a plug-in I use for example

theia.window.createOutputChannel('my-output')

There are show(), hide(), etc methods but the output channel is not seen on the UI until I open the output view and toggle the right output channel.
I was expecting calling show() would have really show the output.

        this.outputChannel = theia.window.createOutputChannel('Yeoman Generator');
        this.outputChannel.show(true);

example of the issue: until the user is not going into the output view, there is no way to know that something was displayed

example with a task: I execute a task, output of the terminal is directly shown