Internal: Models for configuration elements

[Taitava]

(This is about internal refactoring that does not have much visibility to users.)

Currently, all the following settings elements are coded separately, meaning that some of their basic features are created again and again for each of them:

• Shell commands
• Event configurations
• Platform specific command configurations

The list is not yet huge, but the following features will grow it more:

• A modal for asking values from user (Prompt) #17: Creates prompts and prompt fields.
• Custom variables #146: An ability to create custom {{ variables }}.

The common features that are currently done separately each time:

• Configuration handling: storing and reading settings.
• UI: Ability to create new elements (e.g. new shell commands)
• UI: Ability to delete elements (e.g. delete shell commands)

The purpose of this development is to create common code that each of these elements (which I'm calling models) can share. Later, if the functionality gets robust, I could even extract the thing from this repository to create a library that other Obsidian plugins could use, too.