Action to open a web page

[4ian]

There should be an action to open a web page.
For HTML5 games, the action should trigger the opening of a new tab (using window).
For native games, the action should use the system standard way of opening links (a call to system() with start for Windows or xdg-open for Linux, as already done in another action (can't remember its name). Probably an alias to this action would be ok, no need to implement something specific in this case).

[4ian]

Done in c648380