feat(plugin): Replace Unwanted Characters

[nrth3rnlb]

This pull request introduces a new Picard plugin, "Replace Unwanted Characters," which allows users to configure and automate the replacement of unwanted characters in music metadata tags. The plugin provides a user interface for managing affected tags, default and per-tag character mappings, and includes a script function for use in Picard tagger scripts.

Key changes in this pull request:

Plugin core functionality:

• Adds the main plugin logic in __init__.py, including configuration management, character replacement functions, and plugin registration with Picard's plugin system. Also introduces a new $replace_unwanted() script function for use in tagger scripts.

Configuration and defaults:

• Defines default tags, character mapping, and configuration keys in constants.py to support flexible and user-customizable replacements.

User interface:

• Adds a Qt Designer .ui file (replace_unwanted_characters_config.ui) providing a GUI for users to manage affected tags, default mappings, and per-tag overrides.

[nrth3rnlb]

@phw I wanted to reopen the old PR, but somehow it turned into a new PR.

[nrth3rnlb]

Blimey, something's gone wrong.