Add ability to save, edit and load custom practice states

[LukeSaward]

Resolves #122

[oberien]

To have the parsing in the UI element itself, the UI element needs a way to display error messages. In general we seem to be using the pattern quite a lot where we modify the label of an input when parsing the value either onchange or onclick. It might make sense to add that as option to the existing or as new UI-Elements to reduce lots of boilerplate in the future.

I see 3 different ways to achieve this:

• Have the event functions return a string which is added to the label.
• Pass the label to the function and have the label have its own suffix field and function, which gets reset before calling the event function and concatenated afterwards.
• The UI Element could store the old value and reset it every change.

The latter may be the cleanest as it doesn't require a Label to be { text: string, suffix: string }, which might not make much sense. Though it also feels a lot like magic and be unintuitive if the UI Element automatically resets the label all the time.

That change could also be done in a different PR as this one is already quite large.

[oberien]

You never check if the clipboard is valid. This way the user could paste non-number characters and Refunct will crash the next time onclick is called.

[oberien]

This should probably use is_digit.