Only clear site cache when upgrading an active component

[raamdev]

In #145, Quick Cache hooked into upgrader_process_complete to auto-clear the site cache when a component was upgraded. However, it's not always necessary to clear the site cache when upgrading a component:

• Upgrading an inactive plugin should not clear the site cache
• Upgrading an inactive theme should not clear the site cache, unless that theme is the parent theme for a currently active child theme.

[jaswrks]

Example code I have used in the past to check if a plugin is active.

$active_plugins = (is_multisite()) ? wp_get_active_network_plugins() : array();
$active_plugins = array_unique(array_merge($active_plugins, wp_get_active_and_valid_plugins()));

foreach($active_plugins as $active_plugin) // Search.
    if(plugin_basename($active_plugin) === 'bbpress/bbpress.php')
        return true; // bbPress active.

[jaswrks]

I see you chose to use is_plugin_active(). Nice! I didn't know that existed :-)

[raamdev]

Next release changelog:

• Bug Fix: When an active plugin is updated, or when an active theme or the parent theme for an active child theme is updated, or when WordPress Core is updated, Quick Cache now properly clears the cache to ensure that an outdated cache file does not get served to a visitor. See #145 and #327.