CMFileManager: Add summary to 'case-sensitive sorting'

This commit adds a summary text to the 'Use case-sensevite sorting'
checkbox. This makes it much more easy to find a proper translation
in other languages.

Before: http://goo.gl/zkjsV
After:  http://goo.gl/ntL4U

Change-Id: Ie1ea434142049ef89f6d4f98ad15f42f4fbba222