Clean up WebCore tabsToLinks code a little
        https://bugs.webkit.org/show_bug.cgi?id=55606

        No change in behavior, so no tests.

        * html/HTMLFormControlElement.cpp: (WebCore::HTMLFormControlElement::isKeyboardFocusable):
        This is the only caller of tabsToAllControls(). Renamed this function to tabsToAllFormControls().

        * page/EventHandler.cpp:
        (WebCore::EventHandler::isKeyboardOptionTab): Moved from platform specific files. This
        function cannot be file static, because EventHandlerMac.mm uses it.
        (WebCore::eventInvertsTabsToLinksClientCallResult): Merged implementations from platform
        specific files. We can just as well have #if here. Renamed for clarity.
        (WebCore::EventHandler::tabsToLinks): Rewrote in a way that makes it clearer how "invert"
        works.
        
        * page/EventHandler.h: We no longer need invertSenseOfTabsToLinks() here, it's only used
        in EventHandler.cpp....