Themes: Add randomization to composed icon rotation

This patch adds an addition attribute named "plusMinus" which allows
a theme designer to specifiy a +/- variation to the angle of rotation.
The final icon will be rotated by the angle specified +/- a random angle
within the range specified by the value defined with "plusMinus"

ex:
<rotate angle="30" plusMinus="5" />

Change-Id: I51a65c90209547417fe99b215164f8418f9f7f6b