Improve preferences and extension manager

[smathot]

The Preferences and the Plug-in and extension manager aren't very user friendly, in part because they lack search functionality. Three ways to improve this:

• Add a search bar to both options. For the preferences the search should apply to the labels, frame headers, and config values. For the extension manager, the search should apply to the names, descriptions, and values (authors, etc.)
• In the extension manager, there should be a button to open the preferences specifically for that extension.
• The extension manager should look nicer.