feat: add option to copy instead of link on app level

[kassi]

This pull request adds an application specific setting symlink = false|true to application config files.
If this setting is set to false, then no files will be linked, but instead copied over.

In this version, the default value, if the setting does not exist, is true to not break backwards compatibility.

May close: #1854
May obsolete: #2008

Scope
This version allows for copying all files of one application instead of linking them.
It's not meant to work for individual files only, like link filea but do not link fileb. This would add another level of complexity, but also of complication inside the config files, either by adding a tag to each file to sync or have different sections like configuration_files_to_link and configuration_files_to_copy and the same for xdg_configuration_files.
In order to keep it clean and simple, I chose the provided variant.

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• I have followed the Contributing Guidelines
• I have checked to ensure there aren't other open Pull Requests for the same update/change

Improving the Mackup codebase

• My submission passes the tests
• I have linted the code locally prior to submission
• I have written new tests as applicable
• I have added an explanation of what the changes do

[redox]

Thank you for working on this @kassi - looking forward to getting it merged