window: use G_DEFINE_TYPE_WITH_PRIVATE and use a template to define the UI

https://bugzilla.gnome.org/show_bug.cgi?id=705886

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>
window: make headerbar as titlebar and add close button

move the grid in .ui file into the window and
defined that in the .ui file also.

https://bugzilla.gnome.org/show_bug.cgi?id=705886

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>
Bump GTK requirement

it requires for close button in headerbar and for
use template and use G_DEFINE_TYPE_WITH_PRIVATE.

https://bugzilla.gnome.org/show_bug.cgi?id=705886

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>