Refactor manage_plugin_page with objects

Added a new class hierarchy, with one class for each plugin status
(Available, Installed, Invalid):

• PluginForDisplay (abstract parent class)
  • AvailablePlugin
  • InstalledPlugin
  • InvalidPluginForDisplay

Each class takes care of populating column data for display from raw
plugin data as appropriate, and provides an internal method to render
the columns.

This avoids code duplication in the main page, and simplifies the
ListPluginsForDisplay class which now only provides methods to build,
count and print lists of plugins for each status.