Clipper: Add ability to launch clipper window with shortcut (#2272)

* Clipper: Add ability to launch clipper window with shortcut

This change adds a command to the manifest.json file for the web clipper
which launches the webclipper rather than clicking on it.
Because this is a WebExtensions feature and not something homegrown,
 users are able to change (or remove) the shortcut using native browser functionality.

* Add commands for all clipping options

* Remove empty suggestedKeys property from extension manifest

* Add ability to focus the webclipper buttons

* Remove debug log

* Change sendClipMessage warning to error

* Refactor to add a sendContentToJoplin command

* Update index.js

Co-authored-by: Laurent Cozic <laurent22@users.noreply.github.com>