Ugly margins in dialogs since Gtk+-3.14

[Marcus Wichelmann]

Since the update to Gtk+-3.14 the margins of some dialogs are too small. That indication-error is caused by the "Object (use_header_bar: 1);"-workaround to hide the close-buttons.

To fix that remove theese lines:

Object (use_header_bar: 1);
(this.get_header_bar () as Gtk.HeaderBar).show_close_button = false;
this.get_header_bar ().get_style_context ().remove_class ("header-bar");

And replace them with:

this.deletable = false;

(http://valadoc.org/#!api=gtk+-3.0/Gtk.Window.deletable)