RNA/UI: Allow displaying in tooltip why RNA property isn't editable

This patch allows RNA properties to return additional info on their editable state which may then be displayed in tooltips. To use this, set the editable_info callback using RNA_def_property_editable_info_func. It fulfills the same purpose as the editabe callback, but it can return the additional info as string parameter. For generally un-editable properties or properties of a linked data-block, RNA returns default strings.

To test the editable_info callback, this patch uses it for rna_PoseChannel_proxy_editable.