make IsEnabled() return false even if the window parent, and not the window itself...

make IsEnabled() return false even if the window parent, and not the window itself, is disabled and added IsThisEnabled() implementing the old IsEnabled() behaviour; also significantly simplify the window state management code in all ports by factoring out the common parts in wxWindowBase

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@45031 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775