Support for background-image: none

The Style Sheet implementation does not support the standard CSS syntax "background-image: none" for removing background images. Qt's style sheet parser only allows background-image when the value is a Url and doesn't process the "none" value.

I had a look at the style sheet parser code in qcssparser.cpp. It only sets background-image when it detects a `Value::Uri`, but doesn't have any handling for the "none" value.

Workaround:
Use "background-image: url();" in the CSS.

Proposed Solution:
Modify the style sheet parser in qcssparser.cpp to recognize "none" as a valid value for background-image and handle it appropriately by clearing any existing background image. Clarify this in the documentation (style sheets reference).