[NUI][AT-SPI] Promote DoAction implementation from CustomView to View

[NUI][AT-SPI] Promote DoAction implementation from CustomView to View

This is a follow-up to #5109 that enables controls derived from View,
but not CustomView, that use the ViewAccessibilityMode.Custom (e.g.
FluxView), to customize their behaviour related to the AT-SPI Action
interface.

The implementation of AccessibilityDoAction() and its helper methods
is moved up from CustomView to View. Note that the events that
AccessibilityDoAction invokes have already been declared at the View
level, in ViewAccessibilityEvent.cs (e.g. AccessibilityActivated), so
they do not have to be moved anywhere in this PR.