FindGTK2: Use GTK_XXX_LIBRARY_DEBUG libraries in debug mode

If the GTK_XXX_LIBRARY_DEBUG library is available, it is now used when
linking in debug mode XXX.
A new set of variables GTK_XXX_LIBRARY_RELEASE is added and the
original GTK_XXX_LIBRARY uses the optimized/debug syntax.
FindGTK2: Append _LIBRARY to var name in _GTK2_FIND_LIBRARY

This avoids to get the base name using a regexp, in order to use
SelectLibraryConfigurations
FindGTK2: Append _INCLUDE_DIR to var name in _GTK2_FIND_INCLUDE_DIR

This is to keep _GTK2_FIND_INCLUDE_DIR and _GTK2_FIND_LIBRARY coherent
after previous commit
FindGTK2: Update local changelog
FindGTK2: Remove GTK2_SKIP_MARK_AS_ADVANCED option

Variables are now automatically marked as advanced by
SelectLibraryConfigurations
FindGTK2: Do not include a standard module inside a function
Merge topic 'FindGTK2Debug' into next

4b97e8e FindGTK2: Do not include a standard module inside a function
eb4cdd9 FindGTK2: Remove GTK2_SKIP_MARK_AS_ADVANCED option
fd23d6b FindGTK2: Update local changelog
940d733 FindGTK2: Append _INCLUDE_DIR to var name in _GTK2_FIND_INCLUDE_DIR
4e02a62 FindGTK2: Append _LIBRARY to var name in _GTK2_FIND_LIBRARY
e5e995d FindGTK2: Use GTK_XXX_LIBRARY_DEBUG libraries in debug mode