<rdar://problem/6438271> Improve backspace handling of accented characters

        Test: platform/mac/editing/deleting/backward-delete.html

        This change makes WebKit behave like AppKit on the Mac. Other platforms are not affected,
        because the current behavior matches their standards (tested on Windows, assumed elsewhere).

        * dom/Position.cpp:
        (WebCore::Position::previous):
        (WebCore::Position::next):
        (WebCore::Position::uncheckedPreviousOffsetForBackwardDeletion):
        * dom/Position.h:
        Added a new option for previous(), used to move to a next position for backward deletion.
        Renamed EUsingComposedCharacters to PositionMoveType.

        * editing/TypingCommand.cpp: (WebCore::TypingCommand::deleteKeyPressed):
        Use previous(BackwardDeletion) to find a proper range to delete. Also, simplified a check
        surrounding this code a little.

        * editing/htmlediting.cpp:
        (WebCore::nextVisuallyDistinctCandidate):
        (WebCore::previousVisuallyDistinctCandidate):
        Adjusted for renamed enum values.

        * rendering/RenderObject.cpp:
        * rendering/RenderObject.h:
        * rendering/RenderText.h:
        Added previousOffsetForBackwardDeletion().

        * rendering/RenderText.cpp: (WebCore::RenderText::previousOffsetForBackwardDeletion):
        On PLATFORM(MAC), use an algorithm that matches the one AppKit has for backward deletion.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@40710 268f45cc-cd09-0410-ab3c-d52691b4dbfc