Shortcut mapper window: Feature request for "reversed" shortcut search

A premise for a feature

1. Some shortcuts' action is not obvious (like well, what does Ctrl + Shift + L do? eh, nothing changed? does that mean it's free? -- not actually)
2. Another problem is sometimes you want to bind up a group of actions with similar shortcuts (if you dont quite understand what i mean, i can make an example) and while binding a group you can intersect shortcuts with another bind-group which will most probably make you rebind all the previous binds to some another area of "close" or "similar" binds

A feature itself

A way to solve the said problems is to make a "reverse" shortcut finder, which will show you which command is bound (or neither) after passing the keys combination into Shortcut mapper window

For example,

• when i press Ctrl + Alt + S (assuming a default keymap), it will show me a "Save As..." command;

• when i press Ctrl + Shift + Alt + Y (which, i guess, does nothing) it shows me the said key combination is not used for any commands

Start by locating the Shortcut mapper window and reading how it currently displays shortcut assignments. Define the interaction for entering a key combination and showing its bound command or an unused status, including the examples in this issue. Done means the reverse lookup works for assigned and unassigned combinations and is covered by appropriate tests.