widget: Implement hierarchy-level mechanism to claim/deny sequences
The policy of sequence states has been made tighter on GtkGesture, so gestures can never return to a "none" state, nor get out of a "denied" state, a "claimed" sequence can go "denied" though. The helper API at the widget level will first emit GtkWidget::sequence-state-changed on the called widget, and then notify through the same signal to every other widget in the captured event chain. So the effect of that signal is twofold, on one hand it lets the original widget set the state on its attached controllers, and on the other hand it lets the other widgets freely adapt to the sequence state changing elsewhere in the event widget chain. By default, that signal updates every controller on the first usecase, and propagates the default gesture policy to every other widget in the chain on the second. This means that, by default: 1) Sequences start out on the "none" state, and get propagated through all the event widget chain. 2) If a widget in the chain denies the sequence, all other widgets are unaffected. 3) If a widget in the chain claims the sequence, then: 3.1) Every widget below the claiming widget (ie. towards the event widget) will get the sequence cancelled. 3.2) Every widget above the claiming widget that had the sequence as "none" will remain as such, if it was claimed it will go denied, but that should rarely happen. This behavior can be tweaked through the GtkWidget::sequence-state-changed and GtkGesture::event-handled vmethods, although this should be very rarely done.
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