Add a tab preview capture system

[jacobcoughenour]

Implement a system handles taking screen captures of tabs for use in the frontend to preview them.

The previews would be used for:

• switching (previewing their current state)
• history (preview previous or next pages in history)
• reviving dead tabs (preview from right before they were killed)

We'll have to investigate limits to how many previews we keep in memory or store in a cache and for how long.