QmlDesigner: Add image cache

The image cache is saving images and icon of this images in a sqlite
database. If there are no images they are generated in the backgound.
The icons are fetched by item library.

Task-number: QDS-2782
Task-number: QDS-2783
Task-number: QDS-2858
Change-Id: I5a32cccfef7f8fd8eb78902605a09f5da18ce88e
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/gtest-qt-printing.cpp

@@ -85,6 +85,11 @@ std::ostream &operator<<(std::ostream &out, const QTextCharFormat &format)
     return out;
 }
 
+std::ostream &operator<<(std::ostream &out, const QImage &image)
+{
+    return out << "(" << image.width() << ", " << image.height() << ", " << image.format() << ")";
+}
+
 void PrintTo(const QString &text, std::ostream *os)
 {
     *os << text;