User-defined actions ('shortcuts') independent of rules/filters

[cinnamon-rolls]

User's should be able to assign actions to keys independently of rules.

Currently, actions are defined in the rule file, which requires that the user tweaks a pre-existing rule or creates a phony-rule to get special functionality.

Analogous to and inspired by: hydrusnetwork/hydrus#513

Note: The terms 'action' and 'shortcut' are equivalent, and 'rules' = 'filters'