Calculator: Some cleanup

Patch Set 1-2:  * use 'pressed_color' for menu pressed state
                * adjust the 'About' category to be consistent
                  to the already existing one's (e.g. CMFileManager)
                * use '' instead of ""
                * remove unneeded strings

Patch Set 3:    * remove 'operatorDescs'

Patch Set 4-5:  * rebase

SCREENSHOTS
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Before: http://goo.gl/3w4j9
After:  http://goo.gl/BJvGO

Change-Id: Ibb62eaad831e800543da5276c773caa68b6567a7