Save Lua UI scripts separately

Save Lua UI scripts separately

Previously EditorAction and ExitorHook scripts were saved with
instant.xml. The were saved with each session and in the config
dir (for new sessions). This allowed inconsistent UI setups, especially
when loading old sessions that had no or different scripts.

Now Editor scripts (actions and hooks) are saved in a dedicated file,
session-independently. This goes along with ui_config in general
e.g. action-table-columns

The scripts are not saved with ui_config file for two reasons:
ui_config settings related to built-in  ui_config_vars.h,
and in the future there may be further indirection like "ui-rc-file".

Note: previously loaded editor scripts are lost with this change.