feat: add mimetype icons to file tree

[theHamsta]

Try to guess mimetype for file and use it as icon. TODO: check if icon exists and set proper fallback (PS: I have no experience using GIO)

(I'm using KDE, therefore the breeze icons)

[theHamsta]

It's possible to do

        let icon = gio::content_type_get_icon(&mimetype);
        gtk::IconTheme::default().has_gicon(&icon);

but I don't know how to get the icon name without querying specific icon sizes.

[Lyude]

oooh this is a neat idea, I'll try to take a look into this once I get home to see if I can help figure this out (also no problem on the experience, this is my first rust project after all! c:)