Add 16- and 32-pixel images.
    
Append size and scale to all image file names.
Add Buttons test.
Fix button icon drawing.
    
For QPushButton, QTabBar, QToolButton. Also fixes
a couple of other instances of using pixmap size to
calculate layouts.
Fix item view icon/image drawing.
    
Tested with demos/interview and examples/itemviews/dirview,
with QT_HIGHDPI_AWARE=1 set.