SearchView: Add placeholder

[danirabbit]

Fixes #516

Somewhat janky prototype

Todo:

• Search isn't properly cleared when exiting the view
• No obvious way to get back to the grid/list without closing the menu
• Make the items actionable?

[jeremypw]

Definitely need a "Back" button or other way of returning to other view modes (e.g. keep the mode buttons visible like the way AppCenter is going). Also, as we are expanding the information available, why not include the plugins (Calculations and Conversions)?