Device probing for DownloadWidget

[Danilo Cesar Lemes de Paula]

Hi guys,

I've a WIP version of the device probe working for linux and I would
like a third/forth/... opinion (already discussed this subject with Tomaz).

The device probing code for the Gtk version is sitting on
linux.c/macos.c/windows.c and declared in display-gtk.h. It was written
using gtk "helpers" to ease filesystem access, etc.

I've wrote an equivalent method using Qt but, since it's c++ I had to
put it in a new file.

the current code is in my wip branch
git at github.com:danilocesar/subsurface.git wip

Drop some comments here or using the github's review tool[1].

Thanks,

danilo

[1]
https://github.com/danilocesar/subsurface/commit/d368c18da6365336ba82d99374eb5549cca91f94