Add a Linux accessibility backend (AT-SPI 2)#1667
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A closed menu's items have no Components until it opens, so assistive tech could neither discover nor invoke a menu command without first opening the popup. Expose each entry as a zero-size, never-painted child component carrying a menuItem handler - a leaf invokes its command on press, a submenu owner is a focus container holding its entries (recursing). The platform backends then present the menu tree through the standard roles/actions (AT-SPI menu item + action; UIA MenuItem + Invoke/ExpandCollapse; NSAccessibility) with no per-backend code. Entries refresh on menuItemsChanged so menus stay current.
juce_gui_basics has no accessibility implementation on Linux: the generic juce_Accessibility.cpp compiles an empty stub, so the AccessibilityHandler tree that JUCE already maintains is invisible to assistive technologies and UI automation tools. This adds a native backend that exposes that tree as AT-SPI 2 objects over D-Bus (direct D-Bus, as Qt does; not ATK). - No new dependencies: libdbus is loaded with dlopen and its (ABI-stable) types are declared locally, matching how juce_gui_basics treats its other optional Linux libraries. - All bus traffic is handled on the message thread: the bus socket fd is added to the event loop via LinuxEventLoop::registerFdCallback, so the connection is pumped on read-readiness with no polling, no extra thread and no locking. - One fallback object serves the whole /org/a11y/atspi subtree. Implemented interfaces: Accessible, Component, Application, Action, Value, Selection, Table, TableCell, Text, EditableText, Cache, the freedesktop Properties and Introspectable interfaces, plus AT-SPI event signals (state/children/bounds/ text/value changes, focus and window events). - Component.GetAccessibleAtPoint hit-testing (via AccessibilityHandler:: getChildAt), canonical role names from GetRoleName, the Selection interface derived from the children's selected/selectable states (mirroring the UIA SelectionProvider), and postAnnouncement emitting the AT-SPI live-region Announcement signal (at-spi2-core >= 2.46). Tested on Ubuntu 24.04 (at-spi2-core 2.52) with busctl and a libatspi client driving a running application: tree walking with roles/names/states/bounds, coordinate-free invocation via Action.DoAction, hit-testing, value get/set, and Cache.GetItems.
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juce_gui_basics has no accessibility implementation on Linux: the generic juce_Accessibility.cpp compiles an empty stub, so the AccessibilityHandler tree that JUCE already maintains is invisible to assistive technologies and UI automation tools. This adds a native backend that exposes that tree as AT-SPI 2 objects over D-Bus (direct D-Bus, as Qt does; not ATK).
Tested on Ubuntu 24.04 (at-spi2-core 2.52) with busctl and a libatspi client driving a running application: tree walking with roles/names/states/bounds, coordinate-free invocation via Action.DoAction, value get/set, and Cache.GetItems.
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