Shortcuts

This is a plain rebase of @otte's wip/otte/shortcut branch.

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Rebase notes

Dropped

• f9b578fb widget: Remove toplevel flag
• 34b33bf0 widget: Add GtkWidgetClass:root and :unroot functions
• 0a066898 root: Add GtkRoot:get_display vfunc
• a658b220 widget: Add a GtkWidget::root property and gtk_widget_get_root()

Changed

• a5611d22 entry: Port bindings to use shortcuts → move changes to GtkText