[GTK] Refactor GTK's accessibilitity code to be more modular

[GTK] Refactor GTK's accessibilitity code to be more modular
https://bugs.webkit.org/show_bug.cgi?id=76783

Reviewed by Martin Robinson.

New files for the implementation of the AtkEditableText interface,
containing the related code from WebKitAccessibleWrapperAtk.cpp.

* accessibility/gtk/WebKitAccessibleInterfaceEditableText.cpp: Added.
(core):
(webkitAccessibleEditableTextInterfaceInit):
(webkitAccessibleEditableTextSetRunAttributes):
(webkitAccessibleEditableTextSetTextContents):
(webkitAccessibleEditableTextInsertText):
(webkitAccessibleEditableTextCopyText):
(webkitAccessibleEditableTextCutText):
(webkitAccessibleEditableTextDeleteText):
(webkitAccessibleEditableTextPasteText):
* accessibility/gtk/WebKitAccessibleInterfaceEditableText.h: Added.
* accessibility/gtk/WebKitAccessibleWrapperAtk.cpp: Remove code
related to the implementation of the AtkEditableText interface.

Add new files to build files.

* GNUmakefile.list.am: Add WebKitAccessibleInterfaceEditableText.[h|cpp].
* WebCore.gypi: Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@105722 268f45cc-cd09-0410-ab3c-d52691b4dbfc