Added WP Active Plugins In System Status

Although there is list of plugins which were installed but will be informative if number of plugins is displayed before getting list. This will count and display the number of plugin installed and used in the environment section.
Better Text-Domain maintained...
Semantic script adapted

@mikejolly I have used id `#status` instead of class `wc_status_table`. Look this `jQuery( '#status thead, #status tbody' )....` and `<table class="wc_status_table widefat" cellspacing="0" id="status">`. Thus will not harm anything. Additionally, I have adapted the sementatic jQuery script here.
Fix some duplicates entry.
Fix for Unnecessary spaces used
Using Time Constants to set transient

Rather that '60*60*12` we can use `DAY_IN_SECONDS` which is equivalent of `60*60*24`. Let's use one day rather than (1/2)day as it seems better.
Time constant in theme set_transient too...
Merge pull request #6096 from axistools/active-plugins

System Status report page