Align keybindings-widget with vscode

Fixes #5544

The purpose of the PR is to align the behavior and look of the
keybindings-widget more closely with that of vscode.

Features present in the PR:
1. removed the `command id` column.
2. renamed the `scope` column to `source`.
3. display the command id for commands without a label.
4. re-order the columns to that of vscode (actions, command, keybinding, source).
5. prioritized the display of commands with keybindings before those without.
6. updated the documentation.

Signed-off-by: Vincent Fugnitto <vincent.fugnitto@ericsson.com>