Having a property which has the same name as its class leads to confusing
situations (e.g. `BaseButton` has a `shortcut` property of type `Shortcut`
which has a `shortcut` property of type `InputEvent`).
Also renames `is_event` to `matches_event`, and `is_valid` to `has_valid_event`
to better reflect what the methods check.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
