Deserialize KeyboardShortcut from string

**Is your feature request related to a problem? Please describe.**

Rather than having to describe all the fields, it would be nice if one could deserialize from "Ctrl+B" etc. And also serialize to this format. This especially holds for e.g. TOML where the user a bit more easily can write things (rather than e.g. RON).

**Describe the solution you'd like**

As above.

Start by locating the KeyboardShortcut type and reading its current serialization and deserialization behavior. Compare the desired "Ctrl+B" representation with the existing field-based form, including how it should work for TOML and RON; done means both parsing from and serializing to the shortcut string are supported.

Written by the indexing model from the issue text.