Brief description

Attribute definitions have prepared dialog like qargparse have.

Description

First step to use Attribute definitions instead of qargparse in the code. Prepared dialog for attribute definitions to show qargparse like dialog. Loaders can use attribute definitions in get_options (or options) so the dialog is used in loaders. TVPaint loaders are using it. Also update loaders to implement get_options instead of using options so defaults from settings are properly propagated to options.

Additional info

Attribute definitions don't have available all options as qargparse had so we'll have to add missing implementations in future.

Why to replace qargparse?

Module qargparse is Qt based so it must have available Qt binding to be able import and create objects which will be problematic for detached UI workflow.

Testing notes:

1. TVPaint loaders should be able to show options in loader
2. Confirmed options dialog should do what did