file-chooser: Add "directory" option

[hadess]

Add a new directory option to "OpenFile" to allow selecting directories
instead of files.

[hadess]

WIP for #455

[matthiasclasen]

One thing I wondered is: would it be nicer to make this a separate OpenDirectory method ?

[hadess]

One thing I wondered is: would it be nicer to make this a separate OpenDirectory method ?

I wondered that as well, but apart from the file filters, you'd probably want pretty much all the same options there. Really up to you, at the end of the day, it'd be hidden behind the GtkFileChooserNative API for app developers. Maybe @grulja strongly prefers a separate method?

[grulja]

One thing I wondered is: would it be nicer to make this a separate OpenDirectory method ?

I wondered that as well, but apart from the file filters, you'd probably want pretty much all the same options there. Really up to you, at the end of the day, it'd be hidden behind the GtkFileChooserNative API for app developers. Maybe @grulja strongly prefers a separate method?

In QFileDialog it's basically same, it's same method, just a different flag so I don't have any preference. The QFileDialog has following flags:
AnyFile, ExistingFile, Directory, ExistingFiles
Which means that even for the "multiple" option, it's just a matter of flag combination.

[hadess]

Hopefully it works, because I still haven't had time to test it...