HdyHeaderGroup: Disconnect from header bar's signals during dispose

The header bars might still emit signals which leads to

** (gnome-games:25525): CRITICAL **: 11:18:29.087: update_decoration_layouts: assertion 'HDY_IS_HEADER_GROUP (self)' failed

** (gnome-games:25525): CRITICAL **: 11:18:29.087: update_decoration_layouts: assertion 'HDY_IS_HEADER_GROUP (self)' failed

or actual crashes.

Fixes parts of #56
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