Add a Linux accessibility backend (AT-SPI 2)

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.

Author: martijn-resolume

modules/juce_gui_basics/juce_gui_basics.cpp

@@ -224,6 +224,7 @@
  #include "native/juce_FileChooser_windows.cpp"
 
 #elif JUCE_LINUX || JUCE_BSD
+ #include "native/accessibility/juce_Accessibility_linux.cpp"
  #include "native/juce_XSymbols_linux.cpp"
  #include "native/juce_DragAndDrop_linux.cpp"