Add OBSERVER_SCALE environment variable for HiDPI support

[essen]

Workaround for https://bugs.erlang.org/browse/ERL-836

[dgud]

I think we can live with that until wxWidgets-3.2 arrives.
I can't test it tough :-) So please take a real look around.

[essen]

Will do!

[essen]

@dgud Do you know how I can scale the title bar of wxMiniFrame windows? Like the one you get when you double click a port in the ports tab. Right now the window is scaled but the title bar isn't and so it only prints me half the title text (the text is scaled, the area isn't).

[dgud]

The miniframe is created by wxwidgets so I don't think we can do much about it.

[essen]

OK then I think this PR is ready.

[dgud]

Thanks, sqaushed and merged.