fix #1068 ugly media layout tablet

fix #1068 ugly media tablet layout

* Removed specific tablet layout
* Removed MultiSelectGridView (now use stock GridView)
* Cleanup / moved code from MediaBrowserActivity to the corresponding Fragment and Adapter
* Fixed grid spacing / margins
* Fixed untranslatable strings

Any chance the dialog could be reworded here? Perhaps simply "Delete selected items?" or "Delete selection?"

![device-2014-11-10-122549](https://cloud.githubusercontent.com/assets/3903757/4980299/170a9230-68ff-11e4-96a4-2c679733b79c.png)