Display usages of different configurable items

[Taitava]

I'll write a longer description later.

There are several configurable items in the settings. such as:

• Shell commands (obviously)
• Custom shells (will be released in 0.19.0)
• Custom variables
• Output wrappers
• Prompts and prompt fields

Sometimes for e.g. custom variables it would be nice to know where they are used. I developed this already, and here's how it looks like:

The usage listing could be used for other items, too. I'll use this discussion to spin off issues when I'm implementing the usage listing in different phases.

Done already:

• Usage listing for custom variables.
• Usage listing in deletion confirmation prompt for all the items mentioned in the first list of this post. This way user will see, which places the deletion will affect.

Do later:

• Usage listings for Custom shells
• Usage listings for Output wrappers
• Usage listings for Prompts