Fixing several issues with keyingsets:

*Add a new idname to keyingsets, keeping name as label-only (using same string for both made lookup fail when using i18n other than english, as it tried to compare an untranslated static string id against a translated RNA name). Also adding a description string (can be helpful with custom keyingsets, imho).
*Fixed a few other bugs related to that area (namely, you can’t deselect current keyingset from the shift-ctrl-alt-I popup menu, and insert/delete key ops were using a rather strange way to get chosen custom keyingset…).
*Fixed UI code so that it always uses (RNA) enum, and simplified menu-creation code.