Prefs search feature #1926

Consider adding a search box to the prefs panel UI.

When text is typed and the Search button clicked:

1. Hide the sidebar.
2. List all prefs on the screen that (partially?) match the given text.
3. Indicate on each pref in the list which subtab group they belong in, to aid future location without resorting to search.
4. Add a 'clear search' link. Just like we do on tabular panels, which cancels the search and restores the sidebar.
5. Bonus marks for using the URL ?q= so that searches can be performed from other tabs.