Improve action and multiple link definition/configuration

[noctuid]

I need to rethink how actions are handle and how related settings for actions are handled (:<action>-message and :<action>-multiple; these should ideally go away). See #35 for initial discussion.

Some other requirements:

• It should be possible to have a default implementation for an action (e.g. kill-new)
• It should be easy to add new actions that default to other actions if no corresponding keyword (there should be both open-internal and browse-url or open-external)
• It should be possible to define a "composite" action; ivy/avy action selection systems support a single action; it should be easily possible to add a :copy/open action, for example

Probably actions should be defined separately with a link-hint-define-action.