Update ThemeRoller UI to reflect changes in default theme

[jaspermdegroot]

The changes in the default theme require the following changes in the ThemeRoller UI:

• Background image (texture): Add option "none" (see also Theme jquery-ui#1384 (comment))
• Shadow: Remove or hide the controls that set the variables that have been dropped (see https://github.com/jzaefferer/jquery-ui/commit/d2eb0c93cfc5fce158ac2af6ca7b0f50760d9b90)

Those changes should only apply when someone is using 1.12+. For older versions the UI should stay as it is.

[jzaefferer]

Can we update "flat" to skip the image and only use the color? That is useful for all jQuery UI versions, isn't it?

As for shadow: The outdated controls are still need for older UI versions, and since ThemeRoller itself isn't version-aware, I think we should, for now, just leave them. Though we could reorder them, so that the options relevant for box-shadow come first. We can also add hints to labels or tooltips about the deprecated options.

[rxaviers]

Can we update "flat" to skip the image and only use the color? That is useful for all jQuery UI versions, isn't it?

Sounds like a good idea. Implemented on #248.