Add additional ways to open the sidebar

[exhuma]

As the title suggests.

• Keyboard Shortcut
• Item in the Page-Action popup
• Extension Button

[exhuma]

Sidebars are not available in extension manifest v3.0 so this feature may already be doomed to disappear before it is even implemented. Putting it on the backlog as it is questionable if it will be implementable in the future.