Cross-browser navigation logic for reflowable view

Reflowable view now uses custom cfiNavigationLogicReflowable to handle different rendering engines (WebKit, Blink, Gecko, Trident) in uniform way.

TODO:
 - Native Launchers/SDK should load new module: cfiNavigationLogicReflowable: 'epub-modules/epub-renderer/src/readium-shared-js/js/views/cfi_navigation_logic_reflowable_view'
 - common code from cfi_navigation_logic.js and cfi_navigation_logic_reflowable_view.js to be extracted to superclass
 - CfiNavigationLogicReflowable.getFirstVisibleElementCfi does not support custom extension introduced in CfiNavigationLogic related to percentage of visibility as this logic is WebKit specific.