Add icon option to ve.ui.TextInputWidget

Changes:

ve.ui.Widget.css
* Add styles for decorated text input widgets and their icon elements

ve.ui.TextInputWidget.js
* Add icon option which adds an icon before input text

Change-Id: Ib48d795391cb5d110e7dc05658d51129792dfc33
Merge "Add icon option to ve.ui.TextInputWidget"