Further enhance the doc browser

[Infernio]

Following on from #628, these are more difficult and hence will be tackled in 311+:

• [ ] Try adding a search bar for searching the contents of a doc as well
  • In HTML mode, can probably delegate to WebView.Find?
• [ ] PDF support is pretty barebones - it would be nice to have a panel for controlling that as well
• [ ] If we double-click (or right click > Readme...) a plugin that has no associated readme, it might be a good idea to try auto-detecting a readme
• [ ] BAIN-installing a readme-plugin combo should associate the readme with the plugin