Refactor presets module

[lavafroth]

Description

This PR involves replacing a bunch of manual implementations for copying, removing, checking extensions and the like with use of built-in python modules. The JSON reading and parsing is also improved to consume less memory.

Type of change

• Bugfix (Change which fixes an issue)
• New feature (Change which adds new functionality)
• Enhancement (Change which slightly improves existing code)
• Breaking change (This change will introduce incompatibility with existing functionality)

Changelog

• Uses the copyfile function from shutil to copy the GTK CSS file to its backup and vice versa.
• Moves the __get_repo_presets inner function outside and allow it to mutate a dictionary passed to it.
• Factors out the common functionality of loading presets into its own method.
• Uses the suffix attribute for pathlib.Path to check for the file extension instead of instead of stringifying it.
• Uses os.makedirs with exist_ok set to True to create directories if absent.
• Reads JSON structure directly from file handles using json.load(file) instead of reading the whole file and then parsing.
• Uses the in keyword to check if a key is in a dictionary instead of explicitly using the get method.

Testing

• I have tested my changes and verified that they work as expected

How to test the changes

No information provided.