legacy: Temporary workaround for #2897. (#2898)

The `:keys()` and `:buttons()` APIs moved from get/set single methods
to properties. It works fine if you use the new or old API, but has
limitations when mixing them. `awful.rules` calls properties in a loop
after checking if it is a function. Thus it triggers the secondary
codepath to try to handle this case.

This codepath was tested with gears.objects based components
(ie. widgets). It was not tested with clients and tags, and it
didn't work because they use `awful.tag.getproperty` and
`awful.client.property.get` instead of `._private` like all
newer components. Those old functions are officially deprecated,
but used by tons of configs and modules ported from v3.5 and thus
still the default way to access Lua properties in our implementation.

This commit adds a `_private` to anything that doesn't have one to
at least make the error stop. It will "mostly" work until a more
complete solution is added. Reverting the 2 PRs that changed this
would delay getting more feedbacks.

lib/gears/object/properties.lua

@@ -161,10 +161,16 @@ function object._legacy_accessors(obj, name, capi_name, is_object, join_if, set_
     magic_obj["get_"..name] = function(self)
         self = is_object and self or obj
 
+        --FIXME v5 all objects should use _private instead of getproperty.
+        if not self._private then
+            self._private = {}
+        end
+
         self._private[name] = self._private[name] or copy_object(
             obj, {}, name, capi_name, is_object, join_if, set_empty
         )
 
+
         assert(self._private[name])
         return self._private[name]
     end