Allow custom actions to see the current keyboard modifiers

Nobody has claimed this yet.

When writing custom actions, I'd like them to be able to tell if, say, the Alt key was pressed while the action was invoked. For example, maybe the action normally puts up a prompt to select some values, but if Alt is held, it reuses the values from the previous time without prompting.

It would also be nice if their name could automatically update based on the current modifiers. For example, in the above case, the action name should be along the lines of "Do Something…" (note the ellipsis) when Alt is not held, but "Do Something" or "Repeat Something" when Alt is held. This would need to be something in the action object, as it affects the name seen _before_ invoking the action.

Start by tracing how custom actions are defined and how the action object exposes its name and invocation state, then inspect where keyboard modifiers are available during action invocation. Done means custom actions can read the current modifiers and update their displayed name before invocation, covering both the normal and Alt-held examples described in the issue.

Written by the indexing model from the issue text.