UI update : 3-states checkboxes + filter app user/system/both

Finally got a good result for the three-states checkboxes !

+ Added three-states checkboxes in main activity
+ Improved accessibility : clickable images are focusable with DPAD in filter frame
+ Added the ability to show apps from user/system/both (new strings added)
+ Added infos about the user/system/both icons in help section
+ Various fixes

![checkboxes](https://f.cloud.github.com/assets/3890441/1030192/dbdacb5a-0ead-11e3-9212-d3e07539e08b.jpg)