Add px in css for frames

In the code generated for frame widgets CSS offsets are used to position sprites.

At till now no coordinates were specified because the default was px.

style="background-position: 15 0"

With different DOCTYPEs it may be necessary to specifically define the units. This can now be done by specifying this configuration variable in wbd.conf:

cssUnit=px

which results in the following code being generated:

style="background-position: 15px 0px"

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