[Linux][A11y] implement AtkEditableText interface (flutter#33955)

* Make FlAccessibleNode derivable

This allows introducing specialized subclasses that implement such a11y
interfaces as AtkEditableText that should not be implemented for the
generic node.

* FlAccessibleNode: add set value, text selection & perform action

Prepare API required for the upcoming AtkEditableText implementation.

* FlViewAccessible: postpone child node creation

This changes the a11y node creation flow so that (unknown) child nodes
are not pre-created when their parent is updated but "pending" child
node IDs are collected for later. Parent-child relationships are
established at the end of update batches when all nodes have been
created.

* Add FlAccessibleTextField that implements AtkEditableText

Solves: flutter/flutter#103191

ci/licenses_golden/licenses_flutter

@@ -2133,6 +2133,9 @@ FILE: ../../../flutter/shell/platform/linux/fl_accessibility_plugin.h
 FILE: ../../../flutter/shell/platform/linux/fl_accessible_node.cc
 FILE: ../../../flutter/shell/platform/linux/fl_accessible_node.h
 FILE: ../../../flutter/shell/platform/linux/fl_accessible_node_test.cc
+FILE: ../../../flutter/shell/platform/linux/fl_accessible_text_field.cc
+FILE: ../../../flutter/shell/platform/linux/fl_accessible_text_field.h
+FILE: ../../../flutter/shell/platform/linux/fl_accessible_text_field_test.cc
 FILE: ../../../flutter/shell/platform/linux/fl_backing_store_provider.cc
 FILE: ../../../flutter/shell/platform/linux/fl_backing_store_provider.h
 FILE: ../../../flutter/shell/platform/linux/fl_basic_message_channel.cc

shell/platform/linux/BUILD.gn

@@ -99,6 +99,7 @@ source_set("flutter_linux_sources") {
   sources = [
     "fl_accessibility_plugin.cc",
     "fl_accessible_node.cc",
+    "fl_accessible_text_field.cc",
     "fl_backing_store_provider.cc",
     "fl_basic_message_channel.cc",
     "fl_binary_codec.cc",
@@ -192,6 +193,7 @@ executable("flutter_linux_unittests") {
 
   sources = [
     "fl_accessible_node_test.cc",
+    "fl_accessible_text_field_test.cc",
     "fl_basic_message_channel_test.cc",
     "fl_binary_codec_test.cc",
     "fl_binary_messenger_test.cc",