GimpFilterTool::get_settings_ui() is no longer needed, replace
it by a simple utility function in gimpfiltertool-settings.c.
Also, use the GimpFilterOptions functions added earlier, and some
random cleanup.
Filters settings used to be serialized and deserialized only
when a filter tool's GUI was shown, too late for the code that
re-runs/re-shows filters with previous values.
Move the entire loading/saving code to gimp-operation-config.c, even
adding/removing the dummy separator item between timestamped automatic
history and manually saved settings. Load the settings automatically
when a settings container is requested, but still trigger saving from
the few places the container is changed in the GUI; could also
automate that later.
This commit also moves all settings of filters that have their own
tools from gimpdir/tool-options/ to gimpdir/filters/. Add compat code
to try the old filename if the new doesn't exist, so files are
migrated automatically.
WIP, but this step already fixes the bug.
