Adding filters in back-end Installation Discover view for 3.x

+ It was very hard to find and install from discover view when having a large number of items. 
+ I have added filters in discover view like we have in other Joomla! view.
+ Here I have attached screen dump of the filters, 

![discover filter](https://f.cloud.github.com/assets/2002921/831300/31ba36f0-f1cb-11e2-921a-17612ca57687.png)

+ **To test this PR**
   + Apply this patch 
   + Try to discover the items in installation/discover view.
   + Try to filter discover list with **any sting** and filter by **location**, **type** and **folder** using select box.