Use CSS for handling empty labels

Objective:
* Use CSS to control the styling of empty labels

Changes:

ve.ui.LabeledElement.js
* Remove emptyHtml static property
* Add/remove ve-ui-labeledElement-empty class when setting label

ve.ui.ToolGroup.css
* Hide empty labels inside popup tool group handles

ve.ui.Widget.css
* Hide empty labels inside icon button widgets

ve.ui.Toolbar.js
* Only apply default catch-all tool group properties if they were previously undefined

ve.ui.IconButtonWidget.js
* Remove emptyHtml static property

Change-Id: Icd3f772942e74b547e926829c181e914182feb75