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This patch adds flat side bar icons according to
http://blog.qt.io/blog/author/didesous/

The flat icons are supposed to be opt-in, via the theme flag
"FlatSideBarIcons=true". It is false by default for the default
theme for now.

Change-Id: I1cbe69d4e138d5d23c0172a374933ac7a4ce8a5b
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>