Show zoom level as a percentage

Integer zoom level will be displayed as percentage (2 dp),
max and min values for slider adjust to account for
percentage calculation.

paper-input element added after slider, and removed editable
property from slider, to allow for user input of zoom
percentage.

Percentage sign added as a label, after paper-input element.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/33
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/194800043