Adds gtk.TreeView example

[rubiojr]

No description provided.

[rubiojr]

@diamondburned any idea what's going on here? the value of a column is missing if I use this.

[diamondburned]

It seems like the dereference here may cause glib.Value to be freed earlier
than it should be, because that throws away the Finalizer. This would be a bug
in pkg/core/glib, I think.

[rubiojr]

@diamondburned started playing with the bindings today. Quite pleasant to use. Nice job!.

Some early feedback:

• Not having to handle/ignore errors when creating widgets or using function that return a native error that you can't really handle is 💯 🍬 (compared to gotk3, that is).
• Function names follow the Go naming conventions better (NewTreeView vs TreeViewNew).
• win.SetChild(&treeView.Widget) surprised me a bit, found Widgets not usable as Widgetter: FontButton and MenuButton gotk4#31 which helped. TreeView in particular has a Cursor function which made things a bit confusing:

./treeview.go:80:14: cannot use treeView (type *gtk.TreeView) as type gtk.Widgetter in argument to win.Window.SetChild:
        *gtk.TreeView does not implement gtk.Widgetter (wrong type for Cursor method)
                have Cursor() (*gtk.TreePath, *gtk.TreeViewColumn)
                want Cursor() *gdk.Cursor

[diamondburned]

PR blocks on a separate GLib issue.

Edit: tracking in diamondburned/gotk4#34.

[diamondburned]

It seems like the dereference here may cause glib.Value to be freed earlier
than it should be, because that throws away the Finalizer. This would be a bug
in pkg/core/glib, I think.

[diamondburned]

Commit e977c64 in gotk4/pkg should fix the glib.Value issue.

[diamondburned]

It seems like using i.SetValue(&iter, COLUMN_NAME, glib.NewValue(name)) for
each row actually works as expected. Something is likely wrong with i.Set's
internals.

[diamondburned]

gotk4 commit 318362c fixed this. It was an issue to do with how C's int type
is only 4 bytes on 64-bit, but Go's is 8.

[rubiojr]

Nice spelunking, that fixed it. Bumped gotk4 also.