Added option to directly open downloaded files in the associated application (without saving them permanently)

[mityax]

Hey there,
I created a Gtk bottom info bar that pops up when a file has been downloaded. It enables users to either open the file directly (it is saved to a tempfile then) or to save it to their downloads folder.

As I had no experience with vala before, I suggest someone experienced does a little clean up and refactoring or even rewrites the whole thing (it's not that much). Also, two things should probably get fixed so the feature works in all environments (I don't know how to do that in vala):

• The downloads directory of the user is assumed to be ~/Downloads, if it doesn't exist, it's created when a file is downloaded. I'm pretty sure there is a better way to get the default download location (line 278)
• The open button in the notification that appears after clicking save in the bottom bar doesn't yet work (line 289ff)

I'd really appreciate this feature to be in upstream as it helps avoiding clutter in the download folder and enables faster workflow when one just wants to view a file and not save it :)

[artemanufrij]

I will check your PR next few days