Moved validation styling to component scope.

[Igor-Vladyka]

Fixes #140 and maybe a few others.

General problem:

Due to internal dirty check of polymer notification logic that prevents notification of computed methods with more than one argument in case when changed value from 'any' to undefined;

Fix steps:

• Removed validation class setup from the checkbox div (now computed property will hold only one argument).
• Changed validation styling to rely on iron-validatable-behavior in the first place.

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[Igor-Vladyka]

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