screenshot: Use header bar in the interactive dialog

Bump GTK+ requirement to 3.10.0 for this.

https://bugzilla.gnome.org/show_bug.cgi?id=724471
screenshot: don't set a title on the header bar
screenshot: don't pack buttons in a size group
screenshot: don't set text-button style class manually

GTK already does this for us.
screenshot: port screenshot dialog to GtkHeaderBar
screenshot: support hidpi scale factor when rendering preview