Callback order

[AUTOMATIC1111]

Description

Adds a way to reorder callbacks.

• For users, a section in settings UI is added where user can pick which callback to run first, in a multi-select dropdown.
• For developers, support for specifying order of callbacks in metadata.ini via After= and Before=.

Example metadata.ini:

[callbacks/swinir/swinir_model.py/ui_settings]
Before = ldsr/ldsr_model.py/ui_settings
After = hypertile/hypertile_script.py/ui_settings

Here, swinir/swinir_model.py/ui_settings, ldsr/ldsr_model.py/ui_settings and hypertile/hypertile_script.py/ui_settings are three callbacks. They will be reordered like this:

1. hypertile/hypertile_script.py/ui_settings
2. swinir/swinir_model.py/ui_settings
3. ldsr/ldsr_model.py/ui_settings

Each callback is identified by its extension's canonical name, its filename, its category (here, it's ui_settings for all of them), and, optionally, by a user-specified name

• calling script_callbacks.on_ui_settings(on_ui_settings, name='mysettings') creates callback swinir/swinir_model.py/ui_settings/mysettings
• calling script_callbacks.on_ui_settings(on_ui_settings) creates callback swinir/swinir_model.py/ui_settings
• calling script_callbacks.on_ui_settings(on_ui_settings) again creates callback swinir/swinir_model.py/ui_settings-1, then swinir/swinir_model.py/ui_settings-2, etc.

You can see identifiers for existing callbacks in settings (see screenshot below).

Screenshots/videos:

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

[AUTOMATIC1111]

Adding this as PR for comments/suggestions.