Plugins list should respect module status

When generating the 'Enabled Plugins' and 'Disabled Plugins' list, the core module should only display a plugin module if it passes a module_exists() check:

if ( module_exists('linktocontent_category') ) {
  ...
}

(This check should probably also run whenever else the plugin modules are used, referenced or processed.)

At present, the plugins list seems to be generated based on some variable or database value that is created/inserted on plugin module installation (via the linktocontent_register function), and does not get cleared until module uninstallation (when it calls linktocontent_unregister()). This is not appropriate, as it causes errors when a plugin module is deactivated but not uninstalled. It should be possible to deactivate a plugin without uninstalling it and thus losing all configuration information.

Cheers

Andy