Fix width on FunctionController property_name

[RussTedrake]

Before: Text descriptions for buttons (FunctionController) were being truncated unnecessarily to the same width as other Controllers that must share real estate with input elements.

This PR sets the css width for FunctionController elements to 100%. I've added to example.html in case it's useful.

Before:

After:

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[RussTedrake]

@googlebot I signed it!

[RussTedrake]

@mrdoob -- might you let me know how to move this forward (or decide it's not wanted?). Many thanks!