ButtonElement: Add 'font-family: inherit;' to support ButtonInputWidgets

The font-family of regular text usually differs from the one of form
elements like <button/> and <input/> – for example, the defaults on
Windows 7 are, respectively, Arial and Tahoma. This is usually not a
problem, but looks jarring when you have two things that look the
same, but with a different font, side-by-side: http://i.imgur.com/Tu6H9zk.png

Change-Id: I32dfe4f3c4c3a84b0e3c22399dffd3ec706bfce4
FieldLayout: Add 'help' config option support to PHP

Rather than displaying the text in a popup that appears after clicking
on the help icon, we place it in the icon's 'title' attribute.

Bug: 72583
Change-Id: Idd38f3b1d237520056a55f925a7160edd4017fb0
Merge "FieldLayout: Add 'help' config option support to PHP"
Merge "ButtonElement: Add 'font-family: inherit;' to support ButtonInputWidgets"