Fuzzy Find icon in the sidebar

### Problem

There's currently no way to get the file finder (fuzzy find) dialog with the mouse. And no way to discover the shortcut in the UI without going to the addons page

### Proposal

Put a search icon in the sidebar that opens the popover?

### Prior Art (Optional)

We do this with sidebar items in Settings

Start with the existing file-finder (fuzzy find) dialog and the sidebar items in Settings, using the proposal and prior-art screenshot as the UI reference. Add a discoverable search icon that opens the file-finder popover, then verify that it works with the mouse and exposes the shortcut in the editor UI.

索引モデルが issue の本文から書いたものです。